Ultrasonic sensor with piezoelectric elements and vibration isolator

ABSTRACT

An ultrasonic sensor includes a transmitting device, receiving devices arranged in an array, and a circuit device. One receiving device is configured as a reference receiving device. The circuit device includes a reference signal generator and first and second synchronous detectors. The reference signal generator generates a reference signal by using a received signal of the reference receiving device. The first synchronous detector performs synchronous detection of a received signal of one of the receiving devices based on the reference signal to detect a distance to an object. The second synchronous detector performs synchronous detection of received signals of the receiving devices except the reference receiving device based on the reference signal to detect a direction of the object.

CROSS REFERENCE TO RELATED APPLICATION

This application is based on and incorporates herein by reference Japanese Patent Applications No. 2007-334112 filed on Dec. 26, 2007, No. 2008-41215 filed on Feb. 22, 2008, No. 2008-101302 filed on Apr. 9, 2008, No. 2008-135579 filed on May 23, 2008, and No. 2008-276998 filed on Oct. 28, 2008.

FIELD OF THE INVENTION

The present invention relates to an ultrasonic sensor constructed with multiple sensor devices arranged in an array.

BACKGROUND OF THE INVENTION

An ultrasonic sensor has been proposed that includes multiple sensor devices arranged in an array. For example, such an ultrasonic sensor detects a position (i.e., distance and direction) of an object by calculating a time interval between transmitting an ultrasonic wave and receiving the ultrasonic wave (i.e., echo) reflected from the object and by calculating a phase difference between the ultrasonic waves received by receiving devices.

In an ultrasonic sensor disclosed in JP-A-2007-170975, a synchronous detection of a received signal is performed on each receiving element to detect a phase difference between ultrasonic waves received by the receiving devices. In such an approach, the Doppler effect can be reduced. However, since the synchronous detection is performed on each receiving element, the ultrasonic sensor needs many processing devices. Accordingly, the ultrasonic sensor is increased in size.

In an ultrasonic sensor, a detection error may be caused by a direct wave that is directly received by a receiving device without being reflected by the object. JP-A-2007-170975 discloses a technique for reducing the detection error caused by the direct wave. According to the technique, an electrical signal having an opposite phase to an ultrasonic wave transmitted by a transmitting device is supplied to a detection circuit, which detects an object based on the ultrasonic wave received by a receiving device. The technique may be effective only when the transmitting device and the receiving device are physically separated from each other.

In an ultrasonic sensor disclosed in JP-A-2000-253496, multiple sensor devices are arranged in an array such that a transmitting device and a receiving device are physically joined together through an adhesive. A detection sensitivity of the ultrasonic sensor can be improved by causing the transmitting device to vibrate strongly so as to increase pressure of the ultrasonic wave transmitted by the transmitting device. However, when the transmitting device vibrates strongly to transmit the ultrasonic wave of high pressure, the vibration of the transmitting device is likely to propagate to the receiving device through the adhesive. The vibration propagation from the transmitting device to the receiving device may cause a detection error. The technique disclosed in JP-A-2007-170975 is not effective for reducing the detection error caused by the vibration propagation.

SUMMARY OF THE INVENTION

In view of the above, it is an object of the present invention to provide an ultrasonic sensor constructed with multiple sensor devices arranged in an array and configured to reduce at least one of a Doppler effect, a vibration propagation, and size.

According to a first aspect of the present invention, an ultrasonic sensor for detecting an object includes a transmitting device, multiple receiving devices, and a circuit device. The transmitting device transmits an ultrasonic wave to the object. Each receiving device receives the ultrasonic wave reflected from the object and outputs a received signal corresponding to the received ultrasonic wave. The receiving devices include a reference receiving device and at least one non-reference receiving device. The circuit device is electrically coupled to the receiving devices and processes the received signals of the receiving devices. The circuit device includes a reference signal generator, a first synchronous detector, and a second synchronous detector. The reference signal generator generates a reference signal by using the received signal of the reference receiving device. The first synchronous detector performs synchronous detection of the received signal of one of the reference receiving device and the non-reference receiving device based on the reference signal to detect a time interval between transmitting and receiving the ultrasonic wave. The first synchronous detector calculates a distance to the object based on the detected time interval. The second synchronous detector performs synchronous detection of the received signal of the non-reference receiving device based on the reference signal to detect a phase difference between the received signals of the reference receiving device and the non-reference receiving device. The second synchronous detector calculates a direction of the object based on the phase difference.

According to a second aspect of the present invention, an ultrasonic sensor for detecting an object includes multiple sensing devices arranged in an array and a circuit device. Each sensing device includes a piezoelectric element and an acoustic matching member having a first surface exposed to space where the object exists and a second surface joined to the piezoelectric element. The circuit device is electrically coupled to the piezoelectric element. One of the sensing devices is configured as a transmitting device that transmits an ultrasonic wave upon reception of a transmission signal outputted from the circuit device. At least another one of the sensing devices is configured as a receiving device that receives the ultrasonic wave reflected from the object and outputs a received signal corresponding to the received ultrasonic wave to the circuit device. The circuit device includes a drive signal generator that outputs a drive signal to the receiving device based on the transmission signal. The drive signal causes the piezoelectric element of the receiving device to vibrate such that a vibration propagated from the transmitting device to the receiving device is cancelled.

According to a third aspect of the present invention, an ultrasonic sensor for detecting an object includes multiple sensing devices arranged in an array and a circuit device. Each sensing device include a piezoelectric element and an acoustic matching member having a first surface exposed to space where the object exists and a second surface joined to the piezoelectric element. The circuit device is electrically coupled to the piezoelectric element. A first one of the sensing devices is configured as a transmitting device that transmits an ultrasonic wave upon reception of a transmission signal outputted from the circuit device. A second one of the sensing devices is configured as a dummy device that detects only a vibration signal corresponding to a vibration propagating from the transmitting device to the dummy device. At least another one of the sensing devices is configured as a receiving device that receives the ultrasonic wave reflected from the object and outputs a received signal corresponding to the received ultrasonic wave to the circuit device. The circuit device subtracts the vibration signal from the received signal of the receiving device to cancel a vibration propagated from the transmitting device to the receiving device.

According to a fourth aspect of the present invention, an ultrasonic sensor for detecting a first object includes a transmitting device, a receiving device, a housing, a vibration damper, and a vibration isolator. The transmitting device transmits an ultrasonic wave to the first object. The transmitting device includes a first piezoelectric element for emitting the ultrasonic wave and a first acoustic matching member having a transmitting surface. The emitted ultrasonic wave propagates through the first acoustic matching member and is transmitted through the transmitting surface to the first object. The first piezoelectric element is configured as a multilayer piezoelectric element including multiple piezoelectric layers and multiple electrode layers interleaved with the piezoelectric layers. The receiving device is arranged in an array with the transmitting device and receives the ultrasonic wave reflected from the first object. The receiving device includes a second piezoelectric element for detecting the reflected ultrasonic wave and a second acoustic matching member having a receiving surface. The reflected ultrasonic wave is received through the receiving surface and propagates through the second acoustic matching member to the second piezoelectric element. The housing has an inner space for accommodating the transmitting device and the receiving device. The housing has a bottom and an opening portion. The housing is mountable to a second object at the opening portion. The vibration damper is interposed between the opening portion of the housing and each of the first and second acoustic matching members to fix the first and second acoustic matching members to the housing. Further, the vibration damper is interposed between the first and second acoustic matching members to reduce a propagation of the ultrasonic wave between the first and second acoustic matching members. The vibration isolator partitions the inner space of the housing and is located between the transmitting device and the receiving device to reduce the propagation of the ultrasonic wave between the transmitting device and the receiving device.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objectives, features and advantages of the present invention will become more apparent from the following detailed description made with check to the accompanying drawings. In the drawings:

FIG. 1A is a diagram illustrating a top view of an ultrasonic sensor according to a first embodiment of the present invention, and FIG. 1B is a diagram illustrating a cross-sectional view taken along line IB-IB of FIG. 1A;

FIG. 2 is a block diagram of a circuit device of the ultrasonic sensor of the first embodiment;

FIG. 3A is a diagram illustrating a top view of an ultrasonic sensor according to a modification of the first embodiment, and FIG. 3B is a diagram illustrating a majority circuit of a circuit device of the ultrasonic sensor of FIG. 3A;

FIG. 4 is a diagram illustrating a top view of an ultrasonic sensor according to another modification of the first embodiment;

FIG. 5A is a partial block diagram of a circuit device of an ultrasonic sensor according to another modification of the first embodiment, and FIG. 5B is a partial block diagram of a circuit device of an ultrasonic sensor according to another modification of the first embodiment;

FIG. 6A is a diagram illustrating a top view of an ultrasonic sensor according to a second embodiment of the present invention, and FIG. 6B is a diagram illustrating a cross-sectional view taken along line VIB-VIB of FIG. 6A;

FIG. 7 is a block diagram of a circuit device of the ultrasonic sensor of the second embodiment;

FIG. 8 is a diagram illustrating a relationship between a noise change rate and a phase difference between a drive signal applied to a receiving device and a vibration noise prorogating from a transmitting device to the receiving device of the ultrasonic sensor of the second embodiment;

FIG. 9A is a diagram illustrating a transmission signal, a vibration noise, and a drive signal of the ultrasonic sensor of the second embodiment, FIG. 9B is a diagram illustrating a relationship between the vibration noise change rate and the number of waves of the drive signal that is applied to the receiving device before start of a propagation of the vibration noise to the receiving device, and FIG. 9C is a diagram illustrating a relationship between the vibration noise change rate and the number of waves of the drive signal that is applied to the receiving device after end of the propagation of the vibration noise to the receiving device;

FIG. 10A is a diagram illustrating magnitudes of the transmission signal, the vibration noise, and the drive signal, and FIG. 10B is a diagram illustrating a relationship between the vibration noise change rate and a ratio of the magnitude of the drive signal to the magnitude of the vibration noise;

FIG. 11 is a partial block diagram of a circuit device of an ultrasonic sensor according to a modification of the second embodiment;

FIG. 12A is a diagram illustrating a top view of an ultrasonic sensor according to a third embodiment of the present invention, and FIG. 12B is a diagram illustrating a cross-sectional view taken along line XIIB-XIIB of FIG. 12A;

FIG. 13 is a block diagram of a circuit device of the ultrasonic sensor of the third embodiment;

FIG. 14A is a diagram illustrating a top view of an ultrasonic sensor according to a fourth embodiment of the present invention, and FIG. 14B is a diagram illustrating a cross-sectional view taken along line XIVB-XIVB of FIG. 14A;

FIG. 15 is a diagram illustrating a cross-sectional view of an ultrasonic sensor according to a modification of the fourth embodiment;

FIG. 16 is a diagram illustrating a cross-sectional view of an ultrasonic sensor according to another modification of the fourth embodiment;

FIG. 17 is a diagram illustrating a cross-sectional view of an ultrasonic sensor according to another modification of the fourth embodiment;

FIG. 18 is a diagram illustrating a cross-sectional view of an ultrasonic sensor according to a fifth embodiment of the present invention;

FIG. 19 is a diagram illustrating a cross-sectional view of an ultrasonic sensor according to a modification of the fifth embodiment;

FIG. 20 is a diagram illustrating a cross-sectional view of an ultrasonic sensor according to another modification of the fifth embodiment;

FIG. 21A is a diagram illustrating a top view of an ultrasonic sensor according to a sixth embodiment of the present invention, and FIG. 21B is a diagram illustrating a top view of an ultrasonic sensor according to a modification of the sixth embodiment;

FIG. 22 is a diagram illustrating a cross-sectional view of an ultrasonic sensor according to a seventh embodiment of the present invention;

FIG. 23 is a diagram illustrating a cross-sectional view of an ultrasonic sensor according to a modification of the seventh embodiment;

FIG. 24 is a diagram illustrating a cross-sectional view of an ultrasonic sensor according to another modification of the seventh embodiment;

FIG. 25 is a diagram illustrating a cross-sectional view of an ultrasonic sensor according to an eighth embodiment of the present invention;

FIG. 26 is a diagram illustrating a cross-sectional view of an ultrasonic sensor according to a ninth embodiment of the present invention;

FIG. 27A is a diagram illustrating a top view of an ultrasonic sensor according to a tenth embodiment of the present invention, and FIG. 27B is a diagram illustrating a cross-sectional view taken along line XXVIIB-XXVIIB of FIG. 27A;

FIG. 28A is a diagram illustrating a top view of an ultrasonic sensor according to an eleventh embodiment of the present invention, and FIG. 28B is a diagram illustrating a cross-sectional view taken along line XXVIIIB-XXVIIIB of FIG. 28A;

FIG. 29A is a diagram illustrating a top view of an ultrasonic sensor according to a twelfth embodiment of the present invention, and FIG. 29B is a diagram illustrating a cross-sectional view taken along line XXIXB-XXIXB of FIG. 29A;

FIG. 30A is a diagram illustrating a top view of an ultrasonic sensor according to a thirteenth embodiment of the present invention, and FIG. 30B is a diagram illustrating a cross-sectional view taken along line XXXB-XXXB of FIG. 30A;

FIG. 31 is a diagram illustrating a top view of an ultrasonic sensor according to a fourteenth embodiment of the present invention;

FIG. 32 is a diagram illustrating a top view of an ultrasonic sensor according to a modification of the fourteenth embodiment of the present invention; and

FIG. 33 is a diagram illustrating a top view of an ultrasonic sensor according to another modification of the fourteenth embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS First Embodiment

An ultrasonic sensor 10 according to a first embodiment of the present invention is described below with reference to FIGS. 1A, 1B, and 2. For example, the ultrasonic sensor 10 can be used as an obstacle sensor mounted on a vehicle. An upper direction of FIG. 1B indicates an outside of the vehicle.

As shown in FIGS. 1A and 1B, the ultrasonic sensor 10 includes four sensor devices 13 p-13 s arranged in an array of two rows and two columns, a circuit device 18 electrically coupled to the sensor devices 13 p-13 s, and a housing 31 for accommodating the sensor devices 13 p-13 s and the circuit device 18.

The sensor device 13 r serves as a transmitting device for transmitting an ultrasonic wave. The sensor device 13 q is located diagonally to the transmitting device 13 r and serves a reference receiving device. The sensor device 13 p is located adjacent to the reference receiving device 13 q in a horizontal direction and serves as a receiving device (i.e., non-reference receiving device). The sensor device 13 r is located adjacent to the reference receiving device 13 q in a vertical direction and serves as a receiving device (i.e., non-reference receiving device).

The ultrasonic sensor 10 is mounted to a bumper 51 of the vehicle and configured to measure a three-dimensional position of an obstacle.

The transmitting device 13 r, the reference receiving device 13 q, and the receiving devices 13 p, 13 r are identical in structure. Here, the structure of the receiving device 13 p is described. The receiving device 13 p includes a piezoelectric element 11 p and an acrostic matching member 12 p joined to the piezoelectric element 11 p. The piezoelectric element 11 p can both emit and detect an ultrasonic wave. The acrostic matching member 12 p receives the ultrasonic wave reflected from the obstacle and allows the received ultrasonic wave to propagate to the piezoelectric element 11 p.

The piezoelectric element 11 p can be, for example, piezoelectric zirconate titanate (PZT). The piezoelectric element 11 p includes a piezoelectric body and a pair of electrodes 14 p, 15 p. The piezoelectric body has a rectangular cylindrical shape and is identical in cross-section to the acoustic matching member 12 p. The electrodes 14 p, 15 p are respectively formed on opposite surfaces of the piezoelectric body in an ultrasonic propagation direction. That is, the piezoelectric body is sandwiched between the electrodes 14 p, 15 p. Specifically, the electrode 14 p is formed on the surface facing the acrostic matching member 12 p. For example, the electrodes 14 p, 15 p can be formed by plating or sputtering of platinum (Pt), copper (Cu), or silver (Ag) or by baking of conductive paste.

A wiring portion 11 a is formed on a side surface of the piezoelectric element 11 p. The wiring portion 11 a is electrically coupled to the electrode 14 p. Further, the wiring portion 11 a is electrically coupled to the circuit device 18 through a wire 19. Thus, the electrode 14 p of the piezoelectric element 11 p is electrically coupled to the circuit device 18 through the wiring portion 11 a and the wire 19. The electrode 15 p of the piezoelectric element 11 p is electrically coupled to the circuit device 18 through another wire 19.

The acoustic matching member 12 p is made of a material having an acoustic impedance that is greater than an acoustic impedance of air and less than an acoustic impedance of the piezoelectric element 11 p. For example, the acoustic matching member 12 p can be made of a high durability resin material such as a polycarbonate resin.

The acoustic matching members 12 p-12 s are arranged such that a distance d between centers of adjacent acoustic matching members is substantially equal to one-half the wavelength of the ultrasonic wave. A vibration damper 41 is interposed between an inner wall of the housing 31 and a side surface of each of the acoustic matching members 12 p-12 s at a position near an exposed surface 12 a of each of the acoustic matching members 12 p-12 s. Further, the vibration damper 41 is interposed between adjacent acoustic matching members 12 p-12 s. In this way, the acoustic matching members 12 p-12 s are fixed by the vibration damper 41 to the housing 31 near an opening of the housing 31. The vibration damper 41 damps (i.e., reduces or eliminates) an unwanted vibration propagating from the bumper 51 to the acoustic matching members 12 p-12 s.

A width w of the cross-section of the acoustic matching member 12 p is substantially equal to or less than one-half the wavelength of the ultrasonic wave in air. A thickness t of the acoustic matching member 12 p is substantially equal to one-quarter the wavelength of the ultrasonic wave in the acoustic matching member 12 p. In such an approach, a standing wave is produced in the acoustic matching member 12 p. Thus, interference and cancellation between an ultrasonic wave entering the acoustic matching member 12 p and an ultrasonic wave reflected at an interface between the piezoelectric element 11 p and the acoustic matching member 12 p can be reduced. As a result, the ultrasonic wave entering the acoustic matching member 12 p can efficiently propagate to the piezoelectric element 11 p.

The circuit device 18 is electrically coupled to an electronic control unit (ECU) of the vehicle. The ECU is not shown in the drawings. When the ultrasonic sensor 10 transmits the ultrasonic wave, the circuit device 18 receives from the ECU a control signal that controls pressure and phase of the ultrasonic wave to be transmitted. The circuit device 18 outputs to the piezoelectric element 11 r of the transmitting device 13 r a voltage signal according to the control signal. When the ultrasonic sensor 10 receives the ultrasonic wave reflected from the obstacle, the circuit device 18 receives from, for example, the piezoelectric element 11 p of the receiving device 13 p a voltage signal corresponding to pressure and phase of the received ultrasonic wave. The circuit device 18 generates a vibration signal according to the voltage signal and outputs the vibration signal to the ECU.

A block diagram of the circuit device 18 is illustrated in FIG. 2. The circuit device 18 includes amplifiers 21 q, 21 p, 21 s, a phase-locked loop (PLL) 22, a distance-information synchronous detector 23, phase-information synchronous detectors 24, 25, and an ultrasonic transmitter 26 a. The amplifiers 21 q, 21 p, 21 s amplify received signals of the reference receiving device 13 q, the receiving device 13 p, and the receiving device 13 s, respectively.

The phase-locked loop 22 includes a band-pass filter (BPF) 22 a, a phase detector (PD) 22 b, a low-pass filter (LPF) 22 c, a voltage-controlled oscillator (VCO) 22 d. The voltage-controlled oscillator 22 d controls a frequency of its output voltage according to its input voltage.

The distance-information synchronous detector 23 performs a synchronous detection of the received signal of the reference receiving device 13 q. The distance-information synchronous detector 23 includes a phase sensitive detector (PSD) 23 a, a low-pass filter (LPF) 23 b, and a distance calculation circuit 23 c. The phase sensitive detector 23 a has a switch. The distance calculation circuit 23 c includes an amplifier 23 d, a comparator 23 e, a time counter (TC) 23 f, and a distance calculator (DC) 23 g. The comparator 23 e compares an output signal of the amplifier 23 d with a reference voltage Vref as a threshold. The distance calculator 23 g calculates a distance of the obstacle based on clock information outputted from the time counter 23 f. The clock information represents the number of clock signals CLK counted by the time counter 23 f.

The phase-information synchronous detector 24 performs a synchronous detection of the received signal of the receiving device 13 p. The phase-information synchronous detector 24 includes a band-pass filter (BPF) 27 p, a first phase sensitive detector (PSD) 24 a, a second phase sensitive detector (PSD) 24 b, a first low-pass filter (LPF) 24 c, a second low-pass filter (LPF) 24 d, and an angle calculator 24 e. The first low-pass filter 24 c processes an output signal of the first phase sensitive detector 24 a. The second low-pass filter 24 d processes an output signal of the second phase sensitive detector 24 b. The first phase sensitive detector 24 a receives a frequency-locked signal from the phase-locked loop 22 and uses the frequency-locked signal as a reference signal to perform the synchronous detection. The second phase sensitive detector 24 b performs the synchronous detection using a reference signal that is phase-shifted by π/2 from the frequency-locked signal.

The phase-information synchronous detector 25 performs a synchronous detection of the received signal of the receiving device 13 s. The phase-information synchronous detector 25 includes a band-pass filter (BPF) 27 s, a first phase sensitive detector (PSD) 25 a, a second phase sensitive detector (PSD) 25 b, a first low-pass filter (LPF) 25 c, a second low-pass filter (LPF) 25 d, and an angle calculator 25 e. The first low-pass filter 25 c processes an output signal of the first phase sensitive detector 25 a. The second low-pass filter 25 d processes an output signal of the second phase sensitive detector 25 b. The first phase sensitive detector 25 a receives the frequency-locked signal from the phase-locked loop 22 and uses the frequency-locked signal as a reference signal to perform the synchronous detection. The second phase sensitive detector 25 b performs the synchronous detection using a reference signal that is phase-shifted by π/2 from the frequency-locked signal.

The ultrasonic transmitter 26 a outputs a transmission signal St to the transmitting device 13 r. Upon receipt of the transmission signal St from the ultrasonic transmitter 26 a, the transmitting device 13 r transmits an ultrasonic wave. At the same time, the ultrasonic transmitter 26 a outputs the transmission signal St to the time counter 23 f.

The ultrasonic sensor 10 detects the obstacle as follows. The detection of the position of the obstacle requires both distance information Dd and phase difference information Dp. The distance information Dd corresponds to a distance between the ultrasonic sensor 10 and the obstacle, and the phase difference information Dp corresponds to a direction (i.e., angle) of the obstacle with respect to the ultrasonic sensor 10. The phase difference information Dp consists of horizontal phase difference information Dph and vertical phase difference information Dpv.

Firstly, the circuit device 18 receives from the ECU the control signal that controls pressure and phase of an ultrasonic wave to be transmitted. In the circuit device 18, the ultrasonic transmitter 26 a outputs the transmission signal St to the piezoelectric element 11 r of the transmitting device 13 r according to the control signal. The piezoelectric element 11 r vibrates according to the transmission signal St and thus emits the ultrasonic wave having the controlled pressure and phase. The emitted ultrasonic wave propagates through the acoustic matching member 12 r and is transmitted to outside of the vehicle through the transmitting surface 12 a of the acoustic matching member 12 r.

The ultrasonic wave transmitted through the transmitting surface 12 a of the acoustic matching member 12 r is reflected from the obstacle. The reflected ultrasonic wave is received through the receiving surfaces 12 a of the acoustic matching members 12 q, 12 p, and 12 s, respectively. The received ultrasonic waves propagate to the piezoelectric elements 11 q, 11 p, and 11 s through the acoustic matching members 12 q, 12 p, and 12 s, respectively. Then, the ultrasonic waves are converted to the received signals (i.e., voltage signals) by the piezoelectric elements 11 q, 11 p, and 11 s, respectively.

The received signals of the receiving devices 13 q, 13 p, and 13 s have different phases. Here, it is assumed that the received signal of the reference receiving device 13 q is given by sin(ωt+α), that the received signal of the receiving device 13 p is given by sin(ωt+β), and that the received signal of the receiving device 13 r is given by sin(ωt+γ). As noted previously, the receiving device 13 p is located adjacent to the reference receiving device 13 q in the horizontal direction, the receiving device 13 r is located adjacent to the reference receiving device 13 q in the vertical direction.

The received signal of the reference receiving device 13 q is amplified by the amplifier 21 p and then inputted to the phase-locked loop 22. In the phase-locked loop 22, the band-pass filter 22 a removes unwanted frequency components from the received signal of the reference receiving device 13 q. An output signal of the band-pass filter 22 a is low-pass filtered by the low-pass filter 22 c and then inputted to the voltage-controlled oscillator 22 d. The phase detector 22 b detects a phase difference between the received signal of the reference receiving device 13 q and an output signal of the voltage-controlled oscillator 22 d. The phase difference is feedback to the voltage-controlled oscillator 22 d. Thus, the output signal of the voltage-controlled oscillator 22 d is synchronized with (i.e., locked to the frequency of) the received signal of the reference receiving device 13 q.

Below, a method of obtaining the distance information Dd is described. In the distance-information synchronous detector 23, the received signal of the reference receiving device 13 q is inputted to the phase sensitive detector 23 a. The phase sensitive detector 23 a is switched between an amplify (x1) and an inverter (x−1) synchronously with the frequency-locked signal received from the phase-locked loop 22. In this way, the phase sensitive detector 23 a performs the synchronous detection by using the frequency-locked signal as a reference signal. As a result of the synchronous detection, a frequency component of the received signal equal to a frequency of the reference signal is converted to a DC component and passes through the low-pass filter 23 b. The other components of the received signal are converted to AC components and thus removed by the low-pass filter 23 b.

In the distance calculation circuit 23 c, an output signal of the low-pass filter 23 b is amplified by the amplifier 23 d and then inputted to the comparator 23 e. The comparator 23 e compares the inputted signal with the reference voltage Vref. If a voltage level of the inputted signal exceeds the reference voltage Vref, the comparator 23 e outputs a receipt signal to the time counter 23 f. The time counter 23 f counts clock signals CLK from when the transmission signal St is received to when the receipt signal is received. Thus, the time counter 23 f can accurately measure a time elapsed from when the ultrasonic wave is transmitted by the transmitting device 13 r to when the ultrasonic wave is received by the reference receiving device 13 q. The time counter 23 f outputs to the distance calculator 23 g the clock information representing the number of clock signals CLK counted by the time counter 23 f. The distance calculator 23 g calculates the distance between the ultrasonic sensor 10 and the obstacle based on the clock information and outputs the distance information Dd corresponding to the calculated distance.

Below, a method of obtaining the horizontal phase difference information Dph is described below. The received signal of the receiving device 13 p, which is located adjacent to the reference receiving device 13 q in the horizontal direction, is amplified by the amplifier 23 d and then inputted to the phase-information synchronous detector 24 via the band-pass filter 27 p. If a signal to noise (S/N) ratio of the received signal of the receiving device 13 p is good, the band-pass filter 27 p can be eliminated.

In the phase-information synchronous detector 24, the received signal of the reference receiving device 13 p is inputted to the first phase sensitive detector 24 a. The first phase sensitive detector 24 a is switched between an amplify (x1) and an inverter (x−1) synchronously with the frequency-locked signal received from the phased-lock loop 22. In this way, the first phase sensitive detector 24 a performs the synchronous detection by using the frequency-locked signal as a reference signal.

Further, in the phase-information synchronous detector 24, the received signal of the reference receiving device 13 p is inputted to the second phase sensitive detector 24 b. The second phase sensitive detector 24 b is switched between an amplify (x1) and an inverter (x−1) synchronously with a π/2-phase-shifted signal that is phase-shifted by π/2 from the frequency-locked signal. In this way, the second phase sensitive detector 24 b performs the synchronous detection by using the π/2-phase-shifted signal as a reference signal.

The synchronous detection performed by the first phase sensitive detector 24 a is described in detail below. In the first phase sensitive detector 24 a, the received signal is given by sin(ωt+β), and the reference signal is given by sin(ωt+α). The following equation is obtained by multiplying the received signal of sin(ωt+β) by the reference signal of sin(ωt+α). sin(ωt+β)sin(ωt+α)=−½ cos(2ωt+α+β)+½ cos(β−α)  (1)

In the above equation (1), a frequency component of the received signal equal to a frequency of the reference signal is converted to a DC component and passes through the low-pass filter 24 c. The other components of the received signal are converted to AC components and removed by the low-pass filter 24 c. That is, the first low-pass filter 24 c removes the first term “−½ cos(2ωt+α+β)” of the equation (1) and passes the second term “½ cos(β−α)”.

Next, the synchronous detection performed by the second phase sensitive detector 24 b is described in detail below. The second phase sensitive detector 24 b performs the synchronous detection using the π/2-phase-shifted signal as a reference signal.

In the second phase sensitive detector 24 b, the received signal is given by sin(ωt+β), and the reference signal is given by cos(ωt+α). The following equation is obtained by multiplying the received signal of sin(ωt+β) by the reference signal of cos(ωt+α). sin(ωt+β)cos(ωt+α)=½ sin(2ωt+α+β)+½ sin(β−α)  (2)

Therefore, the low-pass filter 24 d removes the first term “½ sin(2ωt+α+β)” of the equation (2) and passes the second term “½ sin(β−α)”.

Each of the output signals of the low-pass filters 24 c, 24 d depend on a phase difference “β−α” between the received signal of the reference receiving device 13 q and the received signal of the receiving device 13 p. The output signals of the low-pass filters 24 c, 24 d are inputted to the angle calculator 24 e. The angle calculator 24 e calculates tan(β−α) by using the output signals of the low-pass filters 24 c, 24 d. In such an approach, the phase difference “β−α” between the received signals of the reference receiving device 13 q and the receiving device 13 p can be calculated without influence of amplitudes of the received signals on the phase difference “β−α”. The tan(β−α) is outputted as the horizontal phase difference information Dph.

Below, a method of obtaining the vertical phase difference information is described below. The received signal of the receiving device 13 s, which is located adjacent to the reference receiving device 13 q in the vertical direction, is processed by the phase-information synchronous detector 25 in the same way as the received signal of the receiving device 13 p is processed by the phase-information synchronous detector 24. Thus, a phase difference “γ−α” between the received signals of the reference receiving device 13 q and the receiving device 13 s is calculated so that the angle calculator 25 e can output tan(γ−α) as the vertical phase difference information Dpv.

As described above, the received signal of the reference receiving device 13 q is used as a reference signal to perform the synchronous detections of the received signals of the receiving devices 13 p, 13 s. The distance information Dd, the horizontal phase difference information Dph, and the vertical phase difference information Dpv are obtained by the distance-information synchronous detector 23 and the phase-information synchronous detectors 24, 25. The position of the obstacle is measured based on the obtained information Dd, Dph, and Dpv and distances between the receiving devices 13 q, 13 p, and 13 s. Since each of the receiving devices 13 p, 13 s is located in a direction perpendicular to the reference receiving device 13 q, the ultrasonic sensor 10 can accurately detect the horizontal and vertical phase differences corresponding to the position of the obstacle over a wide range of area.

The use of the reference receiving device 13 q reduces influence of Doppler effect on the phase differences. Therefore, the phase differences can be easily, accurately detected.

Since one of the receiving devices is used as a reference to perform the synchronous detection, a special (specific) reference device for the synchronous detection is not required. Therefore, the circuit device 18 can be simplified in structure and reduced in size.

As described above, according to the first embodiment, the ultrasonic sensor 10 can have the following advantages.

(1) The phase-looked loop 22 generates a reference signal that is locked to the frequency of the received signal of the reference receiving device 13 q. The received signals of the receiving devices 13 p, 13 s are synchronously detected by using the reference signal. The distance information Dd, the horizontal phase difference information Dph, and the vertical phase difference information Dpv are obtained by the distance-information synchronous detector 23 and the phase-information synchronous detectors 24, 25. The position of the obstacle is measured based on the obtained information Dd, Dph, and Dpv and distances between the receiving devices 13 q, 13 p, and 13 s. Since each of the receiving devices 13 p, 13 s is located in the direction perpendicular to the reference receiving device 13 q, the ultrasonic sensor 10 can accurately detect the horizontal and vertical phase differences corresponding to the position of the obstacle over a wide range of area.

Since the influence of Doppler effect on the phase differences is reduced by using the reference receiving device 13 q, the phase differences can be easily, accurately detected.

Since one of the receiving devices is used as a reference to perform the synchronous detection, the circuit device 18 can be simplified in structure and reduced in size.

(2) The reference signal for the synchronous detection is generated by the phase-locked loop 22. In such an approach, even when the received signal of the reference receiving device 13 q is weak, the reference signal can be generated accurately.

(3) The time elapsed from when the ultrasonic wave is transmitted by the transmitting device 13 r to when the ultrasonic wave is received by the reference receiving device 13 q is measured based on the transmission signal St outputted from the ultrasonic transmitter 26 a. The distance to the obstacle is calculated based on the elapsed time. In such an approach, the distance can be accurately measured.

Modification of First Embodiment

The first embodiment described above can be modified, for example, as follows.

In the first embodiment, the reference signal for the synchronous detection is generated by using the phase-locked loop 22. Alternatively, the reference signal can be generated by using another device such as a comparator constructed with an operational amplifier.

In the first embodiment, the distance to the obstacle is calculated by the distance calculation circuit 23 c based on the transmission signal St outputted from the ultrasonic transmitter 26 a. Alternatively, the distance can be calculated based on the received signals of the receiving devices 13 p, 13 s without using the transmission signal St.

In the first embodiment, the transmitting device 13 r is arranged together with the receiving devices 13 q, 13 p, and 13 s in an array. Alternatively, the transmitting device 13 r can be arranged separately from the receiving devices 13 q, 13 p, and 13 s. For example, the transmitting device 13 r can be formed as a separate piece from the receiving devices 13 q, 13 p, and 13 s and placed outside the housing 31.

In the first embodiment, the transmitting device 13 r is located diagonally to the reference receiving device 13 q, and the receiving devices 13 p, 13 s are located in the direction perpendicular to the reference receiving device 13 q. This arrangement can vary depending on an intended use of the ultrasonic sensor 10. For example, the reference receiving device 13 q and the receiving device 13 s can interchange such that the reference receiving device 13 q is located adjacent to the transmitting device 13 r in the horizontal direction.

The ultrasonic sensor 10 can include multiple transmitting devices. For example, in addition to the transmitting device 13 r, the receiving device 13 p can be used as a transmitting device. In this case, pressure of an ultrasonic wave transmitted by the ultrasonic sensor 10 can be increased by simultaneously driving the transmitting devices 13 r, 13 p. Further, ultrasonic transmission interval can be reduced by alternating driving the transmitting devices 13 r, 13 p. Furthermore, even when one of the transmitting devices 13 r, 13 p is broken, the ultrasonic wave can be transmitted by using the other of the transmitting devices 13 r, 13 p. Thus, the ultrasonic sensor 10 can be configured in a redundant manner.

The number and arrangement of the receiving devices can vary depending on the intended use. In an example shown in FIG. 3A, a reference receiving device 61, receiving devices 62-64, and a transmitting device 65 line up in the horizontal direction. In such an approach, the horizontal phase difference information Dph can be accurately obtained. In this case, as shown in FIG. 3B, phase difference information 62 a-64 a respectively obtained from the receiving devices 62-64 can be inputted to a majority circuit. In such an approach, even when there is a variation in the phase difference information 62 a-64 a, the variation can be reduced. Further, if the variation is caused due to a malfunction of any of the receiving devices 62-64, the malfunctioning receiving device can be identified.

In another example shown in FIG. 4, multiple receiving devices 72 and a transmitting device 73 are arranged circularly around a reference receiving device 71. In this case, one receiving device 72 located in a direction to the obstacle detects a phase difference smaller than those detected by any other receiving devices 72. Therefore, the direction of the obstacle can be accurately detected.

The distance-information synchronous detector 23 can obtain the distance information Dd based on the receive signal of a receiving device other than the reference receiving device 13 q. For example, the output signal of the low-pass filter 24 c of the phase-information synchronous detector 24 can be inputted to the distance calculation circuit 23 c so that the distance-information synchronous detector 23 can obtain the distance information Dd based on the receive signal of the receiving device 13 p.

The reference signal for the synchronous detection can be corrected based on sensor signals outputted from sensors mounted on the vehicle. For example, as shown in FIG. 5A, the circuit device 18 can include a speed correction circuit 82. The speed correction circuit 82 adjusts the frequency of the transmission signal St based on a vehicle speed detected by a vehicle speed sensor 81, which is mounted on the vehicle. In such an approach, influence of Doppler effect due to the vehicle speed on the reference signal can be eliminated so that the reference signal can be generated accurately. In another example shown in FIG. 5B, the circuit device 18 includes a temperature correction circuit 84. The temperature correction circuit 84 adjusts the frequency of the transmission signal St based on a temperature outside the vehicle detected by a temperature sensor 83, which is mounted on the vehicle. In such an approach, influence of a sound speed change, due to a temperature change, on the reference signal can be eliminated so that the reference signal can be generated accurately.

Second Embodiment

An ultrasonic sensor 20 according to a second embodiment of the present invention is described below with reference to FIGS. 6A, 6B, and 7. Differences between the first and second embodiments are as follows.

As shown in FIG. 6A, 6B, the ultrasonic sensor 20 includes four sensor devices 13 p-13 s arranged in an array of two rows and two columns, a circuit device 28 electrically coupled to the sensor devices 13 p-13 s, and the housing 31 for accommodating the sensor devices 13 p-13 s and the circuit device 28.

The sensor device 13 r serves as a transmitting device for transmitting an ultrasonic wave. Each of the other sensor devices 13 p, 13 q, 13 s serves as a receiving device for receiving the ultrasonic wave reflected from the obstacle. It is noted that although the sensor device 13 q of the first embodiment serves as a reference receiving device, the sensor device 13 q of the second embodiment serves as merely a receiving device.

The circuit device 28 is electrically coupled to the ECU (not shown) of the vehicle. When the ultrasonic sensor 20 transmits the ultrasonic wave, the circuit device 28 receives from the ECU the control signal that controls pressure and phase of the ultrasonic wave to be transmitted. The circuit device 28 outputs to the piezoelectric element 11 r of the transmitting device 13 r the voltage signal according to the control signal. When the ultrasonic sensor 20 receives the ultrasonic wave reflected from the obstacle, the circuit device 28 receives from, for example, the piezoelectric element 11 q of the receiving device 13 q the voltage signal corresponding to pressure and phase of the received ultrasonic wave. The circuit device 28 generates the vibration signal according to the voltage signal and outputs the vibration signal to the ECU.

The received signals of the receiving device 13 p, 13 q, 13 s are processed by the circuit device 28 in the same way as each other. As an example, the received signal of the receiving device 13 q is processed by the circuit device 28 as follows. A block diagram of the circuit device 28 corresponding to a section for processing the received signal of the receiving device 13 q is illustrated in FIG. 7. Like the circuit device 18 of the first embodiment, the circuit device 28 includes the amplifier 21 q, the phase-locked loop 22, the distance-information synchronous detector 23, and the phase-information synchronous detector 24.

It is noted that the circuit device 28 further includes a drive signal generator 26. The drive signal generator 26 includes the ultrasonic transmitter (UT) 26 a, a phase inverter (PI) 26 b, and a reception controller (RC) 26 c. The ultrasonic transmitter 26 a outputs the transmission signal St to the transmitting device 13 r, and the transmitting device 13 r transmits an ultrasonic wave upon receipt of the transmission signal St. At the same time, the ultrasonic transmitter 26 a outputs the transmission signal St to the time counter 23 f. The phase inverter 26 b outputs to the reception controller 26 c a signal having opposite phase to the transmission signal St. The reception controller 26 c outputs a drive signal Sd to the receiving device 13 q based on the output signal of the phase inverter 26 b. In response to the drive signal Sd, the piezoelectric elements 11 q is driven (i.e., vibrates) such that a vibration propagating from the transmitting device 13 r to the receiving device 13 q can be cancelled (i.e., removed). Further, the reception controller 26 c outputs a cut signal Sc to the amplifier 21 q during a period of time when the drive signal Sd is outputted. While receiving the cut signal Sc, the amplifier 21 q interrupts the received signal of the receiving device 13 q to a subsequent stage (i.e., the phase-locked loop 22 and the phase-information synchronous detector 24) of the circuit device 28.

The ultrasonic sensor 20 detects the obstacle as follows. The detection of the position of the obstacle requires both distance information Dd and phase-difference information Dp.

The circuit device 28 receives from the ECU the control signal that controls pressure and phase of an ultrasonic wave to be transmitted. In the circuit device 28, the ultrasonic transmitter 26 a outputs the transmission signal St to the piezoelectric element 11 r of the transmitting device 13 r according to the control signal. The piezoelectric element 11 r vibrates according to the transmission signal St and thus emits the ultrasonic wave having the controlled pressure and phase. The emitted ultrasonic wave propagates through the acoustic matching member 12 r and is transmitted to outside of the vehicle through the transmitting surface 12 a.

In the drive signal generator 26, the transmission signal St is phase-inverted by the phase inverter 26 b. The phase-inverted signal having opposite phase to the transmission signal St is inputted to the reception controller 26 c. Then, the reception controller 26 c outputs the drive signal Sd to the receiving device 13 q. In response to the drive signal Sd, the piezoelectric element 11 q is driven such that the vibration propagating from the transmitting device 13 r to the receiving device 13 q can be cancelled. Specifically, the drive signal Sd causes the piezoelectric element 11 q to vibrate at opposite phase to a vibration that propagates from the transmitting device 13 r to the receiving device 13 q through the vibration damper 41. The drive signal Sd is described in more detail later.

The ultrasonic wave transmitted through the transmitting surface 12 a of the acoustic matching member 12 r is reflected from the obstacle. The reflected ultrasonic wave is received through the receiving surfaces 12 a of the acoustic matching member 12 q. The received ultrasonic wave propagates to the piezoelectric element 11 q through the acoustic matching members 12 q. Then, the ultrasonic wave is converted to the received signal (voltage signal) by the piezoelectric elements 11 q. Here, it is assumed that the received signal of the reference receiving device 13 q is given by sin(ωt+α).

The received signal of the reference receiving device 13 q is amplified by the amplifier 21 q and then inputted to the phase-locked loop 22. In the phase-locked loop 22, the band-pass filter 22 a removes unwanted frequency components from the received signal. An output signal of the band-pass filter 22 a is low-pass filtered by the low-pass filter 22 c and then inputted to the voltage-controlled oscillator 22 d. The phase detector 22 b detects a phase difference between the received signal and an output signal of the voltage-controlled oscillator 22 d. The phase difference is feedback to the voltage-controlled oscillator 22 d. Thus, the output signal of the voltage-controlled oscillator 22 d is synchronized with (i.e., locked to the frequency of) the received signal.

Below, a method of obtaining the distance information Dd is described. In the distance-information synchronous detector 23, the received signal of the reference receiving device 13 q is inputted to the phase sensitive detector 23 a. The phase sensitive detector 23 a is switched between an amplify (x1) and an inverter (x−1) synchronously with the frequency-locked signal received from the phase-locked loop 22. In this way, the phase sensitive detector 23 a performs the synchronous detection by using the frequency-locked signal as a reference signal. As a result of the synchronous detection, a frequency component of the received signal equal to a frequency of the reference signal is converted to a DC component and passes through the low-pass filter 23 b. The other components of the received signal are converted to AC components and removed by the low-pass filter 23 b.

In the distance calculation circuit 23 c, an output signal of the low-pass filter 23 b is amplified by the amplifier 23 d and then inputted to the comparator 23 e. The comparator 23 e compares the inputted signal with the reference voltage Vref. If the voltage level of the inputted signal exceeds the reference voltage Vref, the comparator 23 e outputs the receipt signal to the time counter 23 f. The time counter 23 f counts clock signals CLK from when the transmission signal St is received to when the receipt signal is received. Thus, the time counter 23 f can accurately measure the time elapsed from when the ultrasonic wave is transmitted by the transmitting device 13 r to when the ultrasonic wave is received by the reference receiving device 13 q. The time counter 23 f outputs to the distance calculator 23 g the clock information representing the number of clock signals CLK counted by the time counter 23 f. The distance calculator 23 g calculates the distance between the ultrasonic sensor 20 and the obstacle based on the clock information and outputs the distance information Dd corresponding to the calculated distance.

Below, a method of obtaining the phase difference information Dp is described below. The received signal of the receiving device 13 q is amplified by the amplifier 21 p and then inputted to the phase-information synchronous detector 24. In the phase-information synchronous detector 24, the received signal is inputted to the first phase sensitive detector 24 a. The first phase sensitive detector 24 a is switched between an amplify (x1) and an inverter (x−1) synchronously with the frequency-locked signal. In this way, the first phase sensitive detector 24 a performs the synchronous detection by using the frequency-locked signal as a reference signal.

Further, in the phase-information synchronous detector 24, the received signal is inputted to the second phase sensitive detector 24 b. The second phase sensitive detector 24 b is switched between an amplify (x1) and an inverter (x−1) synchronously with a π/2-phase-shifted signal that is phase-shifted by π/2 from the frequency-locked signal. In this way, the second phase sensitive detector 24 b performs the synchronous detection by using the π/2-phase-shifted signal as a reference signal.

The synchronous detection performed by the first phase sensitive detector 24 a is described in detail below. Here, it is assumed that the frequency-locked signal outputted from the phase-locked loop 22 is given by sin(ωt+α). In the first phase sensitive detector 24 a, the received signal is given by sin(ωt+β), and the reference signal is given by sin(ωt+α). The following equation is obtained by multiplying the received signal of sin(ωt+β) by the reference signal of sin(ωt+α). sin(ωt+β)sin(ωt+α)=−½ cos(2ωt+α+β)+½ cos(β−α)  (3)

Thus, the first low-pass filter 24 c removes the first term “−½ cos(2ωt+α+β)” of the equation (3) and passes the second term “½ cos(β−α)” of the equation (3).

Next, the synchronous detection performed by the second phase sensitive detector 24 b is described in detail below. The second phase sensitive detector 24 b performs the synchronous detection using the π/2-phase-shifted signal as a reference signal.

In the second phase sensitive detector 24 b, the received signal is given by sin(ωt+β), and the reference signal is given by cos(ωt+α). The following equation is obtained by multiplying the received signal of sin(ωt+β) by the reference signal of cos(ωt+α). sin(ωt+β)cos(ωt+α)=½ sin(2ωt+α+β)+½ sin(β−α)  (4)

Therefore, the low-pass filter 24 d removes the first term “½ sin(2ωt+α+β)” of the equation (4) and passes the second term “½ sin(β−α)” of the equation (4).

Each of the output signals of the low-pass filters 24 c, 24 d depend on a phase “α”. The output signals of the low-pass filters 24 c, 24 d are inputted to the angle calculator 24 e. The angle calculator 24 e calculates tan(β−α) by using the output signals of the low-pass filters 24 c, 24 d. In such an approach, the phase difference “β−α” can be calculated without influence of the amplitude of the received signal on the phase difference “β−α”. The tan(β−α) is outputted as the phase difference information Dp.

In this way, the received signal of the receiving device 13 q is processed by the circuit device 28 to obtain the distance information Dd and the phase difference information Dp. The received signals of the receiving devices 13 p, 13 s are processed by the circuit device 28 in the same way as the received signal of the receiving device 13 q so that the distance information Dd and the phase information Dp can be obtained. Therefore, the position of the obstacle can be measured based on the obtained information Dd, Dp and distances between the receiving devices 13 q, 13 p, 13 s. Since the receiving devices 13 p, 13 q, 13 s are arranged in an array, the ultrasonic sensor 20 can accurately detect the phase differences corresponding to the position of the obstacle over a wide range of area.

Below, the drive signal Sd outputted from the drive signal generator 26 is described in detail with reference to FIGS. 8-10B.

FIG. 8 illustrates a relationship between a noise change rate and a phase difference between the drive signal Sd applied to the receiving device 13 q and a vibration noise Sn prorogating from the transmitting device 13 r to the receiving device 13 p. As can be seen from FIG. 8, when the drive signal Sd has an opposite phase to the vibration noise Sn, the vibration noise Sn is at a minimum. Further, FIG. 8 indicates that when the phase difference between the drive signal Sd and the vibration noise Sn ranges from about 110 degrees to about 250 degrees, the vibration noise Sn is reduced by the drive signal Sd. Therefore, it is preferable that the drive signal Sd be controlled such that the phase difference between the drive signal Sd and the vibration noise Sn ranges from about 110 degrees to about 250 degrees.

As shown in FIG. 9A, the vibration noise Sn has the same number of continuous waves as the transmission signal St. The drive signal Sd can be applied to the receiving device 13 p even before the vibration noise Sn propagates to the receiving device 13 p. Further, the application of the drive signal Sd to the receiving device 13 p can continue for a predetermined time period even after the propagation of the vibration noise Sn to the receiving device 13 p is ended.

FIG. 9B illustrates a relationship between the vibration noise change rate and the number of waves of the drive signal Sd that is applied to the receiving device 13 q before the vibration noise Sn propagates to the receiving device 13 q. As can be seen from FIG. 9B, when at most two waves of the drive signal Sd are applied to the receiving device 13 q before the start of the propagation of the vibration noise Sn to the receiving device 13 q, the vibration noise Sn is effectively reduced by the drive signal Sd. Therefore, it is preferable that at most two waves of the drive signal Sd be applied to the receiving device 13 q before the start of the propagation of the vibration noise Sn to the receiving device 13 q.

FIG. 9C illustrates a relationship between the vibration noise change rate and the number of waves of the drive signal Sd that is applied to the receiving device 13 q after the propagation of the vibration noise Sn to the receiving device 13 q is ended. As can be seen from FIG. 9C, when at most two waves of the drive signal Sd are applied to the receiving device 13 q after the end of the propagation of the vibration noise Sn to the receiving device 13 q, the vibration noise Sn is effectively reduced by the drive signal Sd. Therefore, it is preferable that at most two waves of the drive signal Sd be applied to the receiving device 13 q after the end of the propagation of the vibration noise Sn to the receiving device 13 q.

Further, FIG. 9C indicates that even when the application of the drive signal Sd to the receiving device 13 q is ended at a time when the last wave of the vibration noise vibration Sn propagates to the receiving device 13 q, the vibration noise vibration Sn is not increased. In other words, the last wave of the drive signal Sd can be applied to the receiving device 13 q when the second last wave of the vibration noise Sn propagates to the receiving device 13 q.

FIG. 10A illustrates magnitudes of the transmission signal St, the vibration noise Sn, and the drive signal Sd. As shown in FIG. 10A, a magnitude Sn1 of the vibration noise Sn is smaller than a magnitude St1 of the transmission signal St.

FIG. 10B illustrates a relationship between the vibration noise change rate and a ratio of a magnitude Sd1 of the drive signal Sd to the magnitude Sn1 of the vibration noise Sn. As can be seen from FIG. 10B, when the ratio of the magnitude Sd1 to the magnitude Sn1 ranges from 50 percent to 100 percent, the vibration noise Sn is reduced by the drive signal Sd. Therefore, it is preferable that the ratio of the magnitude Sd1 to the magnitude Sn1 range from 50 percent to 100 percent.

As described above, according to the second embodiment, the ultrasonic sensor 20 can have the following advantages.

(1) The ultrasonic sensor 20 includes the circuit device 28 having the drive signal generator 26. The drive signal generator 26 outputs the drive signal Sd to the piezoelectric elements 11 q, 11 p, 11 s of the receiving devices 13 q, 13 p, 13 s based on the transmission signal St outputted from the transmitting device 13 r. The drive signal Sd causes the piezoelectric elements 11 q, 11 p, 11 s to vibrate at opposite phase to the vibration that propagates from the transmitting device 13 r to the receiving devices 13 q, 13 p, 13 s through the vibration damper 41. Thus, the vibration propagating from the transmitting device 13 r to the receiving devices 13 q, 13 p, 13 s can be cancelled.

(2) It is preferable that the drive signal Sd be controlled such that the phase difference between the drive signal Sd and the vibration noise Sn range from about 110 degrees to about 250 degrees. In such an approach, the vibration noise Sn can be effectively reduced by the drive signal Sd.

It is preferable that at most two waves of the drive signal Sd be applied to the receiving devices 13 p, 13 q, 13 r before the start of the propagation of the vibration noise Sn to the receiving devices 13 p, 13 q, 13 r. In such an approach, the vibration noise Sn can be effectively reduced by the drive signal Sd.

It is preferable that at most two waves of the drive signal Sd be applied to the receiving devices 13 p, 13 q, 13 r after the end of the propagation of the vibration noise Sn to the receiving device 13 q. Further, it is acceptable that the last wave of the drive signal Sd be applied to the receiving device 13 p, 13 q, 13 r when the second last wave of the vibration noise Sn propagates to the receiving device 13 q.

It is preferable that the ratio of the magnitude Sd1 of the drive signal Sd to the magnitude Sn1 of the vibration noise Sn range from 50 percent to 100 percent. In such an approach, the vibration noise Sn can be effectively reduced by the drive signal Sd.

Modification of Second Embodiment

The second embodiment described above can be modified, for example, as follows.

(1) For example, when the receiving device 13 q receives the ultrasonic wave reflected from the obstacle, the acoustic matching member 12 q vibrates accordingly. The piezoelectric elements 11 q may detect not only a first vibration of the acoustic matching member 12 q corresponding to the received ultrasonic wave but also a second vibration of the acoustic matching member 12 q caused by the reverberation of the first vibration. The second vibration detected by the piezoelectric elements 11 q may result in an error. The drive signal generator 26 can be configured to prevent this problem. Specifically, the second vibration is inputted to the drive signal generator 26 through the amplifier 21 q, and the drive signal generator 26 generates the drive signal Sd based on the second vibration.

(2) As shown in FIG. 11, the ultrasonic sensor 20 can include a set of an amplifier 21 m and a dummy device 13 m instead of the drive signal generator 26. The dummy device 13 m merely detects a vibration propagating from the transmitting device 13 r. The vibration detected by the dummy device 13 m is amplified by the amplifier 21 m, and the amplifier 21 m outputs a dummy vibration signal. In the circuit device 28, the dummy vibration signal is subtracted from the received signal of the receiving device 13 q to cancel a vibration propagating from the transmitting device 13 r to the receiving device 13 q. Thus, like the drive signal generator 26, the amplifier 21 m and the dummy device 13 m work in conjunction with each other to cancel the vibration noise.

For example, the dummy device 13 m can be formed by covering the receiving surface 12 a of one receiving device with a covering material (e.g., rubber) that can interrupt the ultrasonic wave. Alternatively, the dummy device 13 m can be formed by placing one receiving device inside the vibration damper 41 near the receiving device 13 q. By the way, even when the receiving device 13 q and the dummy device 13 m have different sensitivities, the difference in sensitivities between the receiving device 13 q and the dummy device 13 m can be corrected by adjusting the amplifiers 21 q, 21 m.

Third Embodiment

An ultrasonic sensor 30 according to a third embodiment of the present invention is described below with reference to FIGS. 12A, 12B, and 13.

As shown in FIGS. 12A and 12B, the ultrasonic sensor 30 includes four sensor devices 13 p-13 s arranged in an array of two rows and two columns, a circuit device 38 electrically coupled to the sensor devices 13 p-13 s, and the housing 31 for accommodating the sensor devices 13 p-13 s and the circuit device 38.

The sensor device 13 r serves as a transmitting device for transmitting an ultrasonic wave. The sensor device 13 q is located diagonally to the transmitting device 13 r and serves a reference receiving device. The sensor device 13 p is located adjacent to the reference receiving device 13 q in a horizontal direction and serves as a receiving device (i.e., non-reference receiving device). The sensor device 13 r is located adjacent to the reference receiving device 13 q in a vertical direction and serves as a receiving device (i.e., non-reference receiving device). That is, like the first embodiment, the ultrasonic sensor 30 has a reference receiving element.

Further, like the second embodiment, the circuit device 38 of the ultrasonic sensor 30 has the drive signal generator 26 that outputs the drive signal Sd and the cut signal Sc.

In summary, the third embodiment corresponds to a combination of the first and second embodiments. Therefore, according to the third embodiment, the ultrasonic sensor 30 can have the advantages described in the first and second embodiments.

Fourth Embodiment

An ultrasonic sensor 110 according to a fourth embodiment of the present invention is described below with reference to FIGS. 14A and 14B. Like the ultrasonic sensors 10-30 described in the preceding embodiments, for example, the ultrasonic sensor 110 can be used as an obstacle sensor mounted on a vehicle.

The ultrasonic sensor 110 includes a transmitting device 111, receiving devices 112 p, 112 q, 112 r, a vibration damper 118, a first absorber 119, a vibration isolator 190, and a housing 131. The transmitting device 111 transmits an ultrasonic wave. The receiving devices 112 p, 112 q, 112 r detect the ultrasonic wave reflected from the obstacle. The vibration damper 118 prevents propagation (i.e., vibration noise) of the ultrasonic wave among the transmitting device 111 and the receiving devices 112 p, 112 q, 112 r. The first absorber 119 protects the transmitting device 111 and the receiving devices 112 p, 112 q, 112 r from external force (impact) applied to the ultrasonic sensor 110. The vibration isolator 190 isolates the transmitting device 111 from the receiving devices 112 p, 112 q, 112 r to prevent propagation of the ultrasonic wave from the transmitting device 111 to the receiving devices 112 p, 112 q, 112 r. The housing 131 is shaped like a box having an opening. The transmitting device 111, the receiving devices 112 p, 112 q, 112 r, the vibration damper 118, the first absorber 119, and the vibration isolator 190 are accommodated in the housing 131.

The receiving devices 112 p, 112 q, 112 r are identical in structure. In an example below, the structure of the receiving device 112 p is described. The receiving device 112 p includes an acoustic matching member 113 p and a piezoelectric element 114 p joined to the acoustic matching member 113 p. The acoustic matching member 113 p receives the ultrasonic wave reflected from the obstacle and allows the ultrasonic wave to propagate to the piezoelectric element 114 p. Thus, the piezoelectric element 114 p detects the ultrasonic wave.

The piezoelectric element 114 p can be made of, for example, piezoelectric zirconate titanate (PZT). The piezoelectric element 114 p includes a piezoelectric body and a pair of electrodes 115 p. The piezoelectric body has a rectangular cylindrical shape and is identical in cross-section to the acoustic matching member 113 p. The electrodes 115 p are respectively formed on opposite surfaces of the piezoelectric body in an ultrasonic propagation direction. That is, the piezoelectric body is sandwiched between the electrodes 115 p. For example, the electrodes 115 p can be formed by plating or sputtering of platinum (Pt), copper (Cu), or silver (Ag) or by baking of conductive paste.

The acoustic matching member 113 p is made of a material having an acoustic impedance that is greater than an acoustic impedance of air and less than an acoustic impedance of the piezoelectric element 114 p. For example, the acoustic matching member 113 p can be made of a high durability resin material such as a polycarbonate resin.

A thickness t of the acoustic matching member 113 p is substantially equal to one-quarter the wavelength of the ultrasonic wave in the acoustic matching member 113 p. In such an approach, a standing wave is produced in the acoustic matching member 113 p. Therefore, interference and cancellation between the ultrasonic wave entering the acoustic matching member 113 p and the ultrasonic wave reflected at an interface between the piezoelectric element 114 p and the acoustic matching member 113 p can be reduced. As a result, the ultrasonic wave entering the acoustic matching member 113 p can efficiently propagate to the piezoelectric element 114 p. It is preferable that a width w of the acoustic matching member 113 p be substantially equal to or less than one-half the wavelength of the ultrasonic wave in air.

The transmitting device 111 includes an acoustic matching member 113 and a multilayer piezoelectric element 116 joined to the acoustic matching member 113. The acoustic matching member 113 has the same structure as the acoustic matching member 113 p.

The multilayer piezoelectric element 116 can be made of, for example, piezoelectric zirconate titanate (PZT). The multilayer piezoelectric element 116 includes a piezoelectric body and a pair of comb electrodes 117. The piezoelectric body has a rectangular cylindrical shape and is identical in cross-section to the acoustic matching member 113. The comb electrodes 117 are formed to the piezoelectric body such that piezoelectric layers are interleaved with electrode layers. The number of layers can vary according to pressure of the ultrasonic wave to be transmitted. As the number of layers increases, pressure of the ultrasonic wave transmitted by the multilayer piezoelectric element 116 increases.

The electrodes 115 p of the piezoelectric element 114 p are electrically coupled to a circuit device through wires 114 a. The comb electrodes 117 of the multilayer piezoelectric element 116 are electrically coupled to the circuit device through wires 117 a. The circuit device is electrically coupled to the ECU (not shown) of the vehicle. Although the circuit device is not shown in the drawings, the ultrasonic sensor 110 can include, for example, one of the circuit devices 18, 28, 38 described in the preceding embodiments.

The acoustic matching members 113, 113 p-113 r are arranged in an array through the vibration damper 118. It is preferable that a distance d between centers of adjacent acoustic matching members be substantially equal to one-half the wavelength of the ultrasonic wave.

The vibration damper 118 is fixed to the opening of the housing 131 to cover receiving surfaces 113 j of the acoustic matching members 113 p-113 r and a transmitting surface 113 s of the acoustic matching member 113. That is, the receiving surfaces 113 j and the transmitting surface 113 s are not exposed outside the housing 131. The vibration damper 118 prevents foreign matters such as water and dust from entering inside the housing 131. Therefore, reliability of the ultrasonic sensor 110 can be improved. The housing 131 is mounted to the vehicle such that the acoustic matching members 113, 113 p-113 r can face outside the vehicle. For example, the housing 131 is mounted to the bumper 51 of the vehicle.

The vibration damper 118 is made of a material that has a damping constant higher than a damping constant of each of the acoustic matching members 113, 113 p-113 r and that has an acoustic impedance less than an acoustic impedance of each of the acoustic matching members 113, 113 p-113 r. For example, the vibration damper 118 can be made of silicone rubber. Also, the vibration damper 118 can be made of a material having a low elastic modulus and a low density. For example, a foam material such as resin foam, foam rubber, or sponge rubber can be suitably used as a material for the vibration damper 118. Since the vibration damper 118 made of such a material is located among the acoustic matching members 113, 113 p-113 r, the propagation of the ultrasonic wave among the acoustic matching members 113, 113 p-113 r can be prevented. Accordingly, vibration noise originating from the propagation can be prevented.

In the fourth embodiment, the vibration damper 118 has the thickness of one millimeter (i.e., 1 mm) or less at a portion covering the receiving surfaces 113 j and the transmitting surface 113 s. In such an approach, the ultrasonic wave can be suitably transmitted and received through the vibration damper 118.

The first absorber 119 is made of a material having an elastic modulus less than an elastic modulus of each of the piezoelectric element 114 p and the multilayer piezoelectric element 116. For example, the first absorber 119 can be made of a high-polymer material. Specifically, the first absorber 119 can be made of soft resin such as urethane, or a potting material such as rubber or silicon, or the like. The first absorber 119 is located between the housing 131 and each of the multilayer piezoelectric element 116 of the transmitting device 111, the piezoelectric element 114 p of the receiving device 112 p, and piezoelectric elements (not shown) of the receiving devices 112 q, 112 r. Each piezoelectric element is surrounded by the first absorber 119.

Even when impact force is applied to the transmitting device 111 and the receiving devices 112 p-112 r, for example, by a small stone hit against the vibration damper 118 during running of the vehicle, the first absorber 119 absorbs the impact force. Further, the first absorber 119 helps prevent the transmitting device 111 and the receiving devices 112 p-112 r from being displaced toward a bottom 131 a of the housing 131. In this way, the first absorber 119 protects the transmitting device 111 and the receiving devices 112 p-112 r from the impact force. Further, since each piezoelectric element is surrounded by the first absorber 119, each piezoelectric element can be surely protected from environmental factors such as water and dust. Accordingly, the reliability of the ultrasonic sensor 110 can be improved.

The vibration isolator 190 is shaped like a plate and made of a material having a higher elastic modulus and a higher acoustic impedance than the first absorber 119. The vibration isolator 190 is located between the transmitting device 111 and each of the receiving devices 112 p, 112 r, which are located adjacent to the transmitting device 111. The vibration isolator 190 stands on the bottom 131 a of the housing 131 to partition an inner space of the housing 131. As shown in FIG. 14A, the transmitting device 111 is enclosed with the vibration isolator 190 and a side wall of the housing 31. Thus, the transmitting device 111 is isolated by the vibration isolator 190 from the receiving devices 112 p-112 r. The vibration isolator 190 is fixed to the vibration damper 118 at one end and fixed to the first absorber 119 at the other end. The thickness of the vibration isolator 190 is determined to suitably reduce the propagation of the ultrasonic wave from the multilayer piezoelectric element 116 to the acoustic matching members 113 p-113 r. Further, the thickness of the vibration isolator 190 is determined to suitably reduce interference of the vibration isolator 190 with the acoustic matching members 113 p-113 r at the vibration damper 118.

The ultrasonic sensor 110 detects the obstacle as follows. Firstly, the circuit device of the ultrasonic sensor 110 receives from the ECU of the vehicle a control signal for controlling pressure and phase of an ultrasonic wave to be transmitted. The circuit device outputs a voltage signal to the multilayer piezoelectric element 116 according to the control signal. The multilayer piezoelectric element 116 vibrates according to the control signal so as to emit the ultrasonic wave having the controlled pressure and phase.

In the fourth embodiment, the multilayer piezoelectric element 116 has a five-layered structure. Therefore, pressure of the ultrasonic wave emitted by the multilayer piezoelectric element 116 can be five times greater than that of the ultrasonic wave emitted by a single-layer piezoelectric element. Thus, the multilayer piezoelectric element 116 can emit the ultrasonic wave of high pressure.

The vibration isolator 190, which isolates the transmitting device 111 from the receiving devices 112 p-112 r, has the higher elastic modulus and the higher acoustic impedance than the first absorber 119. Therefore, the ultrasonic wave emitted by the multilayer piezoelectric element 116 is reflected at an interface between the first absorber 119 and the vibration isolator 190. In this way, although pressure of the ultrasonic wave emitted by the multilayer piezoelectric element 116 is high, the vibration isolator 190 can suitably reduce the propagation of the ultrasonic wave from the transmitting device 111 to the receiving devices 112 p-112 r. Accordingly, the vibration noise originating from the propagation can be reduced.

The ultrasonic wave emitted by the multilayer piezoelectric element 116 propagates through the acoustic matching member 113 and is then transmitted through the transmitting surface 113 s. The transmitted ultrasonic wave is reflected from the obstacle and received through the receiving surfaces 113 j of the acoustic matching members 113 p-113 r. For example, the ultrasonic wave received through the receiving surface 113 j of the acoustic matching member 113 p propagates through the acoustic matching member 113 p to the piezoelectric element 114 p and is converted to a voltage signal by the piezoelectric element 114 p. The piezoelectric element 114 p outputs the voltage signal to the circuit device. The circuit device processes the voltage signal and outputs the processed signal to the ECU.

For example, the distance between the ultrasonic sensor 110 and the obstacle can be measured based on a time elapsed from when the ultrasonic wave is transmitted to when the ultrasonic wave is received. Further, since the receiving devices 112 p-112 r are arranged in an array, the position of the obstacle can be detected based on a time difference or a phase difference between the ultrasonic waves received by the receiving devices 112 p-112 r.

The vibration damper 118 is interposed among the acoustic matching members 113 p-113 r. The ultrasonic wave is divided by the acoustic matching members 113 p-113 r. The divided ultrasonic waves propagate to the receiving devices 112 p-112 r through the acoustic matching members 113 p-113 r, respectively. Therefore, good crosstalk characteristics are achieved so that the ultrasonic sensor 110 can accurately detect the ultrasonic wave.

Modification of Fourth Embodiment

The fourth embodiment can be modified, for example, as follows. The acoustic matching members 113, 113 p-113 r can be fixed by the vibration damper 118 to the housing 131 at side surfaces near the receiving surfaces 113 j and the transmitting surface 113 s such that the receiving surfaces 113 j and the transmitting surface 113 s are exposed outside. In this case, a coating or the like can be applied to the exposed receiving surfaces 113 j and transmitting surface 113 s.

The vibration isolator 190 can be integrally formed with the housing 131. That is, the housing 131 and the vibration isolator 190 can be formed as a single piece. In such an approach, the number of parts of the ultrasonic sensor 110 can be reduced, and the vibration isolator 190 can be accurately positioned in the housing 131.

As shown in FIG. 15, the vibration damper 118, the housing 131, and the vibration isolator 190 can be joined together by a joint 118 a. In such an approach, the transmitting device 111 and the receiving devices 112 p-112 r can be accurately positioned with respect to the housing 131 and the vibration isolator 190. For example, the joint 118 a can be formed by a two-color (two-material) molding method, a thermocompression bonding method, a laser welding method, a vulcanization bonding method, an adhesive agent, or the like.

The vibration isolator 190 can have a smaller thickness on the acoustic matching member side than on the piezoelectric element side. For example, as shown in FIG. 16, the vibration isolator 190 can be trapezoidal in cross section. In such an approach, the interference of the vibration isolator 190 to the acoustic matching members 113 p-113 r at the vibration damper 118 can be reduced, and strength of the vibration isolator 190 can be increased. For another example, the vibration isolator 190 can be formed with two plates joined together such that the vibration isolator 190 can have a step shape in cross section.

The vibration isolator 190 can have a layer structure in which a soft material is sandwiched between solid materials. For example, as shown in FIG. 17, the vibration isolator 190 can be constructed with metal plates 190 b and a resin layer 190 a sandwiched between the metal plates 190 b. In such an approach, even when the ultrasonic wave is not reflected by the metal plates 190 b, the resin layer 190 a damps the ultrasonic wave passing through the metal plates 190 b. Thus, the vibration noise originating from the propagation of the ultrasonic wave from the transmitting device 111 to the receiving devices 112 p-112 r can be reduced.

The shape of the first absorber 119 can vary depending on the intended use. For example, the first absorber 119 can be formed only between the bottom 131 a of the housing 131 and each of the piezoelectric elements of the transmitting device 111 and the receiving devices 112 p-112 r. Alternatively, the inner space of the housing 131 can be filled with the first absorber 119.

The acoustic matching member 113, 113 p-113 r can have a shape other a rectangular cylinder with a squire cross section. For example, the acoustic matching members 113, 113 p-113 r can have a circular cylinder with a circular cross section. In such an approach, unwanted vibrations in the acoustic matching members 113, 113 p-113 r can be suitably reduced.

The number and arrangement of the transmitting device and the receiving devices can vary depending on the intended use. For example, when the ultrasonic sensor 110 is designed to measure the distance, the ultrasonic sensor 110 needs at least one transmitting device and one receiving device. For another example, when the ultrasonic sensor 110 is designed to measure the direction, the ultrasonic sensor 110 needs at least one transmitting device and two receiving devices.

Like the transmitting device 111, the receiving devices 112 p-112 r can have a multilayer structure.

As described above, according to the fourth embodiment, the ultrasonic sensor 110 can have the following advantages.

(1) The transmitting device 111 employs the multilayer piezoelectric element 116. Therefore, the transmitting device 111 can transmit the ultrasonic wave of high pressure. Since the transmitting device 111 is isolated from the receiving devices 112 p-112 r by the vibration isolator 190, the vibration noise originating from the propagation of the ultrasonic wave from the transmitting device 111 to the receiving devices 112 p-112 r can be reduced. Thus, the ultrasonic sensor 110 can achieve both high pressure ultrasonic wave and low vibration noise.

(2) The first absorber 119 protects the transmitting device 111 and the receiving devices 112 p-112 r from the external force applied to the ultrasonic sensor 110. Since the vibration isolator 190 has the higher acoustic impedance than the first absorber 119, the ultrasonic wave emitted by the multilayer piezoelectric element 116 is reflected at the interface between the first absorber 119 and the vibration isolator 190. Thus, even when the emitted ultrasonic wave has high pressure, the vibration isolator 190 can suitably reduce the propagation of the ultrasonic wave from the transmitting device 111 to the receiving devices 112 p-112 r. Accordingly, the vibration noise originating from the propagation can be reduced.

(3) The vibration isolator 190 can be integrally formed with the housing 131. In such an approach, the number of parts of the ultrasonic sensor 110 can be reduced, and the vibration isolator 190 can be accurately positioned in the housing 131.

(4) The vibration isolator 190 can have a smaller thickness on the acoustic matching member side than on the piezoelectric element side. In such an approach, the interference of the vibration isolator 190 to the acoustic matching members 113 p-113 r at the vibration damper 118 can be reduced, and the strength of the vibration isolator 190 can be increased.

(5) The vibration isolator 190 can have a layer structure in which a soft material is sandwiched between solid materials. In such an approach, even when the ultrasonic wave emitted by the transmitting device 111 is not reflected by the solid materials, the soft material damps the ultrasonic wave passing through the solid materials. Thus, the vibration noise originating from the propagation of the ultrasonic wave can be reduced.

(6) The transmitting surface 113 s and the receiving surfaces 113 j are covered with the vibration damper 118. In such an approach, the transmitting device 111 and the receiving devices 112 p-112 r can be surely protected from environmental factors such as water and dust. Accordingly, the reliability of the ultrasonic sensor 110 can be improved.

Fifth Embodiment

An ultrasonic sensor 120 according to a fifth embodiment of the present invention is described below with reference to FIG. 18. Differences between the fourth and fifth embodiments are as follows.

As shown in FIG. 18, the ultrasonic sensor 120 has a second absorber 121 in addition to the first absorber 119. The second absorber 121 is made of a material having a lower elastic modulus and a lower acoustic impedance than the first absorber 119. For example, the second absorber 121 can be made of gel. The first absorber 119 is placed between the bottom 131 a of the housing 131 and each of the piezoelectric elements of the transmitting device 111 and the receiving devices 112 p-112 r. The second absorber 121 is placed on the first absorber 119 to surround the piezoelectric elements of the transmitting device 111 and the receiving devices 112 p-112 r.

Even when the external force is applied to the transmitting device 111 and the receiving devices 112 p-112 r through the vibration damper 118, the first absorber 119 absorbs the external force. Since each piezoelectric element is surrounded by the second absorber 121, each piezoelectric element can be surely protected from environmental factors such as water and dust. Accordingly, the reliability of the ultrasonic sensor 110 can be improved. Further, since the second absorber 121 placed near the acoustic matching members 113, 113 p-113 r has the lower elastic modulus than the first absorber 119, the damping of the ultrasonic wave in the acoustic matching members 113, 113 p-113 r can be reduced.

Modification of Fifth Embodiment

The fifth embodiment can be modified, for example, as follows. An absorber placed between the bottom 131 a of the housing 131 and each of the receiving devices 112 p-112 r can have an elastic modulus different than that of an absorber placed between the bottom 131 a and the transmitting device 111. Specifically, as shown in FIG. 19, a third absorber 122 can be placed between the multilayer piezoelectric element 116 and the bottom 131 a. The third absorber 122 has a higher elastic modulus than the first absorber 119. In such an approach, the multilayer piezoelectric element 116 can effectively emit the ultrasonic wave.

Further, as shown in FIG. 20, the third absorber 122 can have a greater height, measured from the bottom 131 a, than the first absorber 119. In such an approach, a lateral vibration of the multilayer piezoelectric element 116 is restricted so that the multilayer piezoelectric element 116 can effectively emit the ultrasonic wave.

As described above, according to the fifth embodiment, the ultrasonic sensor 120 can have the following advantages.

(1) The ultrasonic sensor 120 has the second absorber 121 in addition to the first absorber 119. The second absorber 121 has the lower elastic modulus than the first absorber 119. The first absorber 119 is placed between the bottom 131 a of the housing 131 and each of the piezoelectric elements of the transmitting device 111 and the receiving devices 112 p-112 r. The second absorber 121 is placed on the first absorber 119 to surround the piezoelectric elements of the transmitting device 111 and the receiving devices 112 p-112 r. The external force applied to the transmitting device 111 and the receiving devices 112 p-112 r through the vibration damper 118 can be absorbed by the first absorber 119. The piezoelectric elements are surrounded by the second absorber 121 so that the piezoelectric elements can be protected from environmental factors such as water and dust. Since the second absorber 121 has the lower elastic modulus than the first absorber 119, the damping of the ultrasonic wave in the acoustic matching members 113, 113 p-113 r can be reduced.

(2) The third absorber 122 can be placed between the multilayer piezoelectric element 116 and the bottom 131 a. Since the third absorber 122 has the higher elastic modulus than the first absorber 119, the multilayer piezoelectric element 116 can effectively emit the ultrasonic wave.

(3) The third absorber 122 can have the height greater than that of the first absorber 119. In such an approach, the lateral vibration of the multilayer piezoelectric element 116 is restricted so that the multilayer piezoelectric element 116 can effectively emit the ultrasonic wave.

Sixth Embodiment

An ultrasonic sensor 130 according to a sixth embodiment of the present invention is described below with reference to FIG. 21A. Differences between the fourth and sixth embodiments are as follows.

As shown in FIG. 21A, the ultrasonic sensor 130 includes two transmitting devices 111 a, 111 b and two receiving devices 112 q, 112 r. The receiving devices 112 q, 112 r are arranged side by side in the horizontal direction. The transmitting devices 111 a, 111 b are located below the receiving devices 112 q, 112 r and arranged side by side in the horizontal direction. The transmitting devices 111 a, 111 b are isolated from the receiving devices 112 q, 112 r by the vibration isolator 190 so that the propagation of the ultrasonic wave from the transmitting devices 111 a, 111 b to the receiving devices 112 q, 112 r can be reduced.

An acoustic matching member 113 a of the transmitting device 111 a has a rectangular cross section having a length (i.e., long side) in the horizontal direction and a width (i.e., short side) in a vertical direction. Therefore, the transmitting device 111 a has a larger directivity in the vertical direction.

On the other hand, an acoustic matching member 113 b of the transmitting device 111 b has a rectangular cross section having a length in the vertical direction and a width in the horizontal direction. Therefore, the transmitting device 111 b has a larger directivity in the horizontal direction.

In summary, the transmitting devices 111 a, 111 b have different directivities. The ultrasonic sensor 130 can switch between the transmitting devices 111 a, 111 b according to road conditions, for example. In the sixth embodiment, the transmitting devices 111 a, 111 b alternately transmit the ultrasonic wave. When the transmitting device 111 a transmits the ultrasonic wave, the ultrasonic sensor 130 can detect the obstacle near the ground (i.e., road surface). When the transmitting device 111 b transmits the ultrasonic wave, the ultrasonic sensor 130 can detect the obstacle over a wide horizontal range of area.

When the road conditions are bad, the ultrasonic wave reflected from the road surface is large. Therefore, when the road conditions are bad, the ultrasonic sensor 130 may not detect the obstacle based on the ultrasonic wave transmitted by the transmitting device 111 a, which has the vertical directivity. In such a case, the ultrasonic sensor 130 may detect the obstacle based on the ultrasonic wave transmitted by the transmitting device 111 b, which has the horizontal directivity. Conversely, the ultrasonic sensor 130 may detect the obstacle based on the ultrasonic wave transmitted by the transmitting device 111 a, when the ultrasonic sensor 130 cannot detect the obstacle based on the ultrasonic wave transmitted by the transmitting device 111 b. In this way, the transmitting devices 111 a, 111 b work complementarily to each other so that the ultrasonic sensor 130 can detect the obstacle with high sensitivity over a wide range of area.

Modification of Six Embodiment

The sixth embodiment can be modified, for example, as follows. As shown in FIG. 21B, the receiving devices 112 q, 112 r can have different directivities. An acoustic matching member 113 q of the receiving device 112 q has a rectangular cross section having a length in the horizontal direction and a width in the vertical direction. Therefore, the receiving device 112 q has a larger directivity in the vertical direction. On the other hand, an acoustic matching member 113 r of the receiving device 112 r has a rectangular cross section having a length in the vertical direction and a width in the horizontal direction. Therefore, the receiving device 112 r has a larger directivity in the horizontal direction. The ultrasonic sensor 130 can switch between the receiving devices 112 q, 112 r according to road conditions, for example.

The number and arrangement of the transmitting devices and receiving devices can vary depending on the intended use.

As described above, according to the ultrasonic sensor 130 of the sixth embodiment, a set of transmitting devices or a set of receiving devices have different directivities. Therefore, the ultrasonic sensor 130 can detect the obstacle with high sensitivity over a wide range by changing the directivities according to conditions such as road conditions.

Seventh Embodiment

An ultrasonic sensor 140 according to a fourth embodiment of the present invention is described below with reference to FIG. 22. Differences between the fourth and seventh embodiments are as follows.

As shown in FIG. 22, the ultrasonic sensor 140 has a vibration isolator 191 instead of the vibration isolator 190. The vibration isolator 191 includes a first plate 191 a, a second plate 191 b, and a core layer 191 c sandwiched between the first and second plates 191 a, 191 b. Each of the first and second plates 191 a, 191 b has a higher acoustic impedance than the core layer 191 c.

Therefore, an acoustic impedance difference is large at an interface between the first plate 191 a and the core layer 191 c and at an interface between the second plate 191 b and the core layer 191 c. Thus, the ultrasonic wave from the transmitting device 111 is easily reflected at each interface so that the propagation of the ultrasonic wave from the transmitting device 111 to the receiving devices 112 p-112 r can be reduced.

Modification of Seventh Embodiment

The seventh embodiment can be modified, for example, as follows. As shown in FIG. 23, the ultrasonic sensor 140 can have a vibration isolator 192 instead of the vibration isolator 191. The vibration isolator 192 includes a first plate 192 a and a second plate 192 b. The first plate 192 a and the second plate 192 b are joined together such that a sealed space 192 c can be provided between the first plate 192 a and the second plate 192 b. The sealed space 192 c is filled with a sealed member 192 d. Each of the first and second plates 192 a, 192 b has a higher acoustic impedance than the sealed member 192 d. For example, as shown in FIG. 23, the first plate 192 a has projections that are in contact with the second plate 192 b to provide the sealed space 192 c.

In such an approach, an acoustic impedance difference can be large at an interface between the first plate 192 a and the sealed member 192 d and at an interface between the second plate 192 b and the sealed member 192 d. Thus, the ultrasonic wave from the transmitting device 111 is easily reflected at each interface so that the propagation of the ultrasonic wave from the transmitting device 111 to the receiving devices 112 p-112 r can be reduced. For example, the sealed member 192 d can be gel or gas such as air. Gas has high compressibility, and gel has large damping constant. Therefore, the sealed member 192 d can suitably absorb the propagation of the ultrasonic wave from the transmitting device 111 to the receiving devices 112 p-112 r.

The vibration isolator 192 can further include a honeycomb member (not shown) placed in the sealed space 192 c to partition the sealed space 192 c. In such an approach, the propagation of the ultrasonic wave from the transmitting device 111 to the receiving devices 112 p-112 r can be efficiently reduced. Further, strength of the vibration isolator 192 can be increased.

Alternatively, as shown in FIG. 24, the ultrasonic sensor 140 can have a vibration isolator 193 instead of the vibration isolator 191. The vibration isolator 193 has a trapped gas bubble 193 a. An acoustic impedance difference in the vibration isolator 193 can be large at the trapped gas bubble 193 a. Therefore, the ultrasonic wave from the transmitting device 111 is easily reflected at the trapped gas bubble 193 a so that the propagation of the ultrasonic wave from the transmitting device 111 to the receiving devices 112 p-112 r can be reduced. The vibration isolator 193 can be easily manufactured compared to the other vibration isolators. Therefore, manufacturing cost of the ultrasonic sensor 140 can be reduced.

As described above, according to the seventh embodiment, the ultrasonic sensor 140 can have the following advantages.

(1) The vibration isolator 191 includes the first plate 191 a, the second plate 191 b, and the core layer 191 c sandwiched between the first and second plates 191 a, 191 b. Each of the first and second plates 191 a, 191 b has the higher acoustic impedance than the core layer 191 c. Therefore, the acoustic impedance difference in the vibration isolator 191 is large at the interface between the first plate 191 a and the core layer 191 c and at the interface between the second plate 191 b and the core layer 191 c. Thus, the ultrasonic wave from the transmitting device 111 is easily reflected at each interface so that the propagation of the ultrasonic wave from the transmitting device 111 to the receiving devices 112 p-112 r can be reduced.

(2) The ultrasonic sensor 140 can have the vibration isolator 192 instead of the vibration isolator 191. The vibration isolator 192 includes the first plate 192 a, the second plate 192 b, and the sealed space 192 c filled with the sealed member 192 d. Each of the first and second plates 192 a, 192 b has the higher acoustic impedance than the sealed member 192 d. The acoustic impedance difference in the vibration isolator 192 can be large at the interface between the first plate 192 a and the sealed member 192 d and at the interface between the second plate 192 b and the sealed member 192 d. Thus, the ultrasonic wave from the transmitting device 111 is easily reflected at each interface so that the propagation of the ultrasonic wave from the transmitting device 111 to the receiving devices 112 p-112 r can be reduced. The sealed member 192 d can be gel or gas such as air. Gas has high compressibility, and gel has large damping constant. Therefore, the sealed member 192 d can suitably absorb the propagation of the ultrasonic wave.

(3) The vibration isolator 192 can further include the honeycomb layer (not shown) placed in the sealed space 192 c to partition the sealed space 192 c. In such an approach, the propagation of the ultrasonic wave from the transmitting device 111 to the receiving devices 112 p-112 can be efficiently reduced, and the strength of the vibration isolator 192 can be increased.

(4) The ultrasonic sensor 140 can have the vibration isolator 193 instead of the vibration isolator 191. The vibration isolator 193 has the trapped gas bubble 193 a. The acoustic impedance difference in the vibration isolator 193 can be large at the trapped gas bubble 193 a. Therefore, the ultrasonic wave from the transmitting device 111 is easily reflected at the trapped gas bubble 193 a so that the propagation of the ultrasonic wave from the transmitting device 111 to the receiving devices 112 p-112 r can be reduced. The vibration isolator 193 can be easily manufactured so that the manufacturing cost of the ultrasonic sensor 140 can be reduced.

Eighth Embodiment

An ultrasonic sensor 150 according to an eighth embodiment of the present invention is described below with reference to FIG. 25. Differences between the fourth and eighth embodiments are as follows.

As shown in FIG. 25, the ultrasonic sensor 150 includes a housing 131 b instead of the housing 131 and a vibration isolator 194 instead of the vibration isolator 190. The housing 131 b has a through hole 131 c on a bottom 131 a. The vibration isolator 194 has an extension 194 a exposed outside the housing 131 b through the through hole 131 c. An elastic member 151 is attached to the extension 194 a to reduce a vibration of the extension 194 a. For example, the elastic member 151 can be made of rubber, gel, or the like.

In such an approach, a resonance frequency of the vibration isolator 194 can be reduced. Further, since the vibration of the extension 194 a is reduced by the elastic member 151, the vibration of the entire vibration isolator 194 is reduced accordingly. Thus, the propagation of the ultrasonic wave from the transmitting device 111 to the receiving devices 112 p-112 r can be reduced.

As described above, according to the eighth embodiment, the ultrasonic sensor 150 can have the following advantages.

(1) The vibration isolator 194 has the extension 194 a exposed outside the housing 131 b through the through hole 131 c. Thus, the resonance frequency of the vibration isolator 194 can be reduced. Further, since the extension 194 a is exposed outside the housing 131 b, the vibration of the vibration isolator 194 can be effectively reduced at the extension 194 a. Thus, the propagation of the ultrasonic wave from the transmitting device 111 to the receiving devices 112 p-112 r can be reduced.

(2) The elastic member 151 is attached to the extension 194 a of the vibration isolator 194. Thus, the vibration of the extension 194 a is reduced by the elastic member 151. The vibration of the entire vibration isolator 194 is reduced accordingly.

Ninth Embodiment

An ultrasonic sensor 160 according to a ninth embodiment of the present invention is described below with reference to FIG. 26. Differences between the fourth and ninth embodiments are as follows.

As shown in FIG. 26, the ultrasonic sensor 160 includes an actuator 161 serving as a noise chancellor. The actuator 161 is placed between the vibration isolator 190 and the bottom 131 a of the housing 131. The actuator 161 is electrically coupled to the circuit device (not shown) through a wire 161 a. The actuator 161 detects the vibration of the vibration isolator 190 and applies a vibration having opposite phase to the detected vibration to the vibration isolator 190, thereby damping the vibration of the vibration isolator 190. Alternatively, the vibration applied by the actuator 161 to the vibration isolator 190 can be adjusted according to pressure of the ultrasonic wave emitted by the transmitting device 111.

The actuator 161 can be placed between the bottom 131 a and each of the receiving devices 112 p-112 r. In this case, the actuator 161 detects vibrations of the receiving devices 112 p-112 r and applies vibrations having opposite phase to the detected vibrations to the receiving devices 112 p-112 r, thereby damping the vibrations of the receiving devices 112 p-112 r.

As described above, according to the ninth embodiment, the ultrasonic sensor 160 includes the actuator 161 placed between the vibration isolator 190 and the bottom 131 a of the housing 131. The actuator 161 detects the vibration of the vibration isolator 190 and applies the vibration having opposite phase to the detected vibration to the vibration isolator 190, thereby damping the vibration of the vibration isolator 190. Thus, the propagation of the ultrasonic wave from the transmitting device 111 to the receiving devices 112 p-112 r can be reduced.

Tenth Embodiment

An ultrasonic sensor 170 according to a tenth embodiment of the present invention is described below with reference to FIGS. 27A and 27B. Differences between the fourth and tenth embodiments are as follows. The ultrasonic sensor 170 has transmitting device 11 and one receiving device 112 p. Alternatively, like the fourth embodiment, the ultrasonic sensor 170 can have three receiving devices 112 p-112 r.

As shown in FIGS. 27A and 27B, the ultrasonic sensor 170 has a vibration isolator 195 instead of the vibration isolator 190. The vibration isolator 195 has uneven side surfaces with recesses and projections.

A resonant frequency of the vibration isolator 195 depends on the shape of the uneven side surfaces. Therefore, resonance of the vibration isolator 195 can be reduced by adjusting the shape of the uneven side surfaces.

As described above, according to the ultrasonic sensor 170 of the tenth embodiment, the vibration isolator 195 has the uneven side surfaces. Since the resonant frequency of the vibration isolator 195 depends on the shape of the uneven side surfaces, the resonance of the vibration isolator 195 can be reduced by adjusting the shape of the uneven side surfaces.

Eleventh Embodiment

An ultrasonic sensor 180 according to an eleventh embodiment of the present invention is described below with reference to FIGS. 28A and 28B. Differences between the fourth and eleventh embodiments are as follows. The ultrasonic sensor 180 has one transmitting device 111 and one receiving device 112 p. Alternatively, like the fourth embodiment, the ultrasonic sensor 180 can have three receiving devices 112 p-112 r.

As shown in FIGS. 28A and 28B, the ultrasonic sensor 180 has a vibration isolator 196 instead of the vibration isolator 190. The vibration isolator 196 has an edge portion 196 a supported by the housing 131. The thickness of the edge portion 196 a is smaller than that of a center portion 196 b of the vibration isolator 196. That is, the vibration isolator 196 is supported by the housing 131 at its edge and thinner at its edge than at its center.

When the ultrasonic wave from the transmitting device 111 propagates to the vibration isolator 196, the center portion 196 b is less likely to vibrate than the edge portion 196 a. Therefore, the propagation of the ultrasonic wave to the receiving device 112 p can be reduced.

As described above, according to the ultrasonic sensor 180 of the eleventh embodiment, the vibration isolator 196 is thinner at the edge portion 196 a supported by the housing 131 than at the center portion 196 b. When the ultrasonic wave from the transmitting device 111 propagates to the vibration isolator 196, the center portion 196 b is less likely to vibrate than the edge portion 196 a. Therefore, the propagation of the ultrasonic wave from the transmitting device 111 to the receiving device 112 p can be reduced.

Twelfth Embodiment

An ultrasonic sensor 180 a according to a twelfth embodiment of the present invention is described below with reference to FIGS. 29A and 29B. Differences between the fourth and twelfth embodiments are as follows. The ultrasonic sensor 180 a has one transmitting device 111 and one receiving device 112 p. Alternatively, like the fourth embodiment, the ultrasonic sensor 180 a can have three receiving devices 112 p-112 r.

As shown in FIGS. 29A and 29B, the vibration isolator 190 of the ultrasonic sensor 180 a has reinforcements 197 on a first side surface facing the receiving device 112 p. For example, the reinforcements 197 are equally spaced from each other. The vibration isolator 190 is reinforced by the reinforcements 197 so that the vibration isolator 190 can have high strength. Alternatively, the vibration isolator 190 can have the reinforcements 197 on a second side facing the transmitting device 111.

The reinforcements 197 can be arranged in a honeycomb structure. In such an approach, the strength of the vibration isolator 190 can be much increased by the reinforcements 197.

As described above, according to the ultrasonic sensor 180 a of the twelfth embodiment, the vibration isolator 190 is reinforced by the reinforcements 197 so that the strength of the vibration isolator 190 can be increased.

The strength of the vibration isolator 190 can be much increased by arranging the reinforcements 197 in a honeycomb structure.

Thirteenth Embodiment

An ultrasonic sensor 180 b according to a thirteenth embodiment of the present invention is described below with reference to FIGS. 30A and 30B. Differences between the fourth and thirteenth embodiments are as follows.

As shown in FIGS. 30A and 30B, the vibration damper 118 of the ultrasonic sensor 180 b has a slit 118 b that is formed on each side of the vibration isolator 190 along the vibration isolator 190. One slit 118 b is located between the vibration isolator 190 and the acoustic matching member 113 of the transmitting device 111. The other slit 118 b is located between the vibration isolator 190 and the acoustic matching members 113 p-113 r of the receiving devices 112 p-112 r.

The vibration from the acoustic matching member 113 of the transmitting device 111 is distributed at the slit 118 b. Thus, the slit 118 b reduces the propagation of the ultrasonic wave from the transmitting device 111 to the receiving devices 112 p-112 r. In FIGS. 30A and 30B, the slit 118 b is formed on each side of the vibration isolator 190. Alternatively, the slit 118 b can be formed on only one side of the vibration isolator 190. Multiple slits 118 b can be formed on each side or one side of the vibration isolator 190. In FIGS. 30A and 30B, the slit 118 b is formed continuously along the vibration isolator 190. Alternatively, the slit 118 b can be formed intermittently along the vibration isolator 190.

As described above, according to the ultrasonic sensor 180 b of the thirteenth embodiment, the vibration damper 118 has the slit 118 b. The slit 118 b is located between the acoustic matching member 113 of the transmitting device 111 and each of the acoustic matching members of the receiving devices 112 p-112 r. Thus, the propagation of the ultrasonic wave from the transmitting device 111 is distributed at the slit 118 b so that the propagation of the ultrasonic wave to the receiving devices 112 p-112 r can be reduced.

Fourteenth Embodiment

An ultrasonic sensor 180 c according to a fourteenth embodiment of the present invention is described below with reference to FIG. 31. Differences between the fourth and fourteen embodiments are as follows.

As shown in FIG. 31, the ultrasonic sensor 180 c includes one transmitting device 111 and two receiving devices 112 p, 112 q. The receiving devices 112 p, 112 q are arranged such that a distance from the receiving device 112 p to the vibration isolator 190 is equal to a distance from the receiving device 112 q to the vibration isolator 190. Further, the receiving devices 112 p, 112 q are arranged such that a distance from the receiving device 112 p to the transmitting device 111 is equal to a distance from the receiving device 112 q to the transmitting device 111.

In such an approach, the propagation of the ultrasonic wave from the transmitting device 111 to the receiving devices 112 p, 112 q occurs at the same time. Therefore, influence of the propagation on the receiving devices 112 p, 112 q can be easily reduced by signal processing.

The ultrasonic sensor 180 c can include three or more receiving devices. For example, as shown in FIG. 32, the ultrasonic sensor 180 c can include four receiving devices 112 p-112 s that are arranged such that a distance from each of the receiving devices 112 p-112 s to the vibration isolator 190 is the same, and such that a distance from each of the receiving devices 112 p-112 s to the transmitting device 111 is the same.

Further, as shown in FIG. 33, the ultrasonic sensor 180 c can have a vibration isolator 198 instead of the vibration isolator 190. The vibration isolator 198 has a honeycomb structure to partition the inner space of the housing 131 into multiple spaces 198 a-198 g. The space 198 a is located adjacent to each of the other spaces 198 b-198 g so that the space 198 a can be located in the center of the spaces 198 a-198 g. The transmitting device 111 is arranged in the center space 198 a. Receivers 112 are respectively arranged in the other spaces 198 b-198 g such that a distance from each receiving device 112 to the transmitting device 111 is the same. Since the vibration isolator 198 has a honeycomb structure, the vibration isolator 198 has high strength.

As described above, according to the ultrasonic sensor 180 c of the fourteenth embodiment, the following advantages can be achieved.

(1) The receiving devices are arranged such that the distance from each receiving device to the vibration isolator 190 is the same, and such that the distance from each receiving device to the transmitting device 111 is the same. In such an approach, the propagation of the ultrasonic wave from the transmitting device 111 to the receiving devices occurs at the same time. Therefore, influence of the propagation on the receiving devices can be easily reduced by signal processing.

The ultrasonic sensor 180 c can have the vibration isolator 198 instead of the vibration isolator 190. The vibration isolator 198 has a honeycomb structure to partition the inner space of the housing 131 into multiple spaces 198 a-198 g. The transmitting device 111 is arranged in the center space 198 a, and the receiving devices are respectively arranged in the other spaces 198 b-198 g in such a manner that a distance from each receiving device to the transmitting device 111 is the same. Since the vibration isolator 198 has a honeycomb structure, the vibration isolator 198 has high strength.

Modifications of Embodiments

The embodiments described above can be modified in various ways. For example, the ultrasonic sensors 10-30, 110-180 c can be mounted to a portion other than the bumper 51 of the vehicle. The ultrasonic sensors 10-30, 110-180 c can be used as an obstacle sensor mounted to a robot.

Such changes and modifications are to be understood as being within the scope of the present invention as defined by the appended claims. 

1. An ultrasonic sensor for detecting a first object, comprising: a transmitting device for transmitting ultrasonic wave to the first object, the transmitting device including a first piezoelectric element for emitting the ultrasonic wave and a first acoustic matching member having a transmitting surface, the emitted ultrasonic wave propagated through the first acoustic matching member and being transmitted through the transmitting surface to the first object, the first piezoelectric element being configured as a multilayer piezoelectric element including a plurality of piezoelectric layers and a plurality of electrode layers interleaved with the plurality of piezoelectric layers; a receiving device, arranged in an array with the transmitting device, for receiving the ultrasonic wave reflected from the first object, the receiving device including a second piezoelectric element for detecting the reflected ultrasonic wave and a second acoustic matching member having a receiving surface, the reflected ultrasonic wave being received through the receiving surface and propagated through the second acoustic matching member to the second piezoelectric element; a housing having an inner space for accommodating the transmitting device and the receiving device, the housing having a bottom and an opening portion, the housing being mountable to a second object at the opening portion; a vibration damper interposed between the opening portion of the housing and each of the first and second acoustic matching members to fix the first and second acoustic matching members to the housing, the vibration damper being interposed between the first and second acoustic matching members to reduce a propagation of the ultrasonic wave between the first and second acoustic matching members; and a vibration isolator for partitioning the inner space of the housing and located between the transmitting device and the receiving device to reduce the propagation of the ultrasonic wave between the transmitting device and the receiving device.
 2. The ultrasonic sensor according to claim 1, further comprising: a first absorber interposed between the bottom of the housing and each of the first and second piezoelectric elements and between an inner side wall of the housing and each of the first and second piezoelectric elements to protect the transmitting device and the receiving device from external force, wherein the vibration isolator has a higher acoustic matching impedance than the first absorber.
 3. The ultrasonic sensor according to claim 1, wherein the vibration isolator is integrally formed with the housing.
 4. The ultrasonic sensor according to claim 1, wherein the vibration isolator has a first thickness on the first and second acoustic matching members side and a second thickness on the first and second piezoelectric elements side, and the first thickness is less than the second thickness.
 5. The ultrasonic sensor according to claim 1, wherein the vibration isolator includes first and second members and a core member that are layered together such that the core member is sandwiched between the first and second members, and each of the first and second members has a higher elastic modulus than the core member.
 6. The ultrasonic sensor according to claim 1, wherein the vibration isolator includes first and second members and a core member that are layered together such that the core member is sandwiched between the first and second members, and each of the first and second members has a higher acoustic impedance than the core member.
 7. The ultrasonic sensor according to claim 1, wherein the vibration isolator includes first and second members that are joined together to provide a sealed space therebetween, and the sealed space is filled with a material having a lower acoustic impedance than those of each of the first and second members.
 8. The ultrasonic sensor according to claim 7, wherein the vibration isolator includes a honeycomb structural body placed in the sealed space to partition the sealed space into a plurality of spaces.
 9. The ultrasonic sensor according to claim 1, wherein the vibration isolator has a plurality of gas bubbles trapped therein.
 10. The ultrasonic sensor according to claim 1, further comprising: a second absorber located on the first absorber to surround the first and second piezoelectric elements, wherein the second absorber has a lower elastic modulus than the first absorber.
 11. The ultrasonic sensor according to claim 10, wherein the first absorber includes a first portion located on the first piezoelectric element side and a second portion located on the second piezoelectric element side, and the first portion of the first absorber has a higher elastic modulus than the second portion of the first absorber.
 12. The ultrasonic sensor according to claim 11, wherein the first portion of the first absorber has a greater height, measured from the bottom of the housing, than the second portion of the first absorber.
 13. The ultrasonic sensor according to claim 1, wherein the transmitting surface of the transmitting device and the receiving surface of the receiving device are covered with the vibration damper.
 14. The ultrasonic sensor according to claim 1, wherein the transmitting device or the receiving device comprises a plurality of devices having different directivities.
 15. The ultrasonic sensor according to claim 1, wherein the bottom of the housing has a through hole, and the vibration isolator has a portion exposed outside the housing through the through hole.
 16. The ultrasonic sensor according to claim 15, wherein the vibration isolator has a vibration inhibitor attached to the exposed portion of the vibration isolator to inhibit the exposed portion from vibrating.
 17. The ultrasonic sensor according to claim 1, further comprising: a vibration damping circuit configured to apply to the vibration isolator a vibration having an opposite phase to a vibration of the vibration isolator to damp the vibration of the vibration isolator.
 18. The ultrasonic sensor according to claim 1, wherein the vibration isolator has an uneven side surface.
 19. The ultrasonic sensor according to claim 1, wherein the vibration isolator is supported by the housing at an edge portion, and the vibration isolator is thinner at the edge portion than at a center portion.
 20. The ultrasonic sensor according to claim 1, wherein the vibration isolator is reinforced with a reinforcing member placed on a side surface.
 21. The ultrasonic sensor according to claim 20, wherein the reinforcing member has a honeycomb structure.
 22. The ultrasonic sensor according to claim 1, wherein the vibration damper has a slit located between the first and second acoustic matching members.
 23. The ultrasonic sensor according to claim 1, wherein the receiving device comprises a plurality of receiving devices, and a distance from each receiving device to the transmitting device is the same, and a distance from each receiving device to the vibration isolator is the same.
 24. The ultrasonic sensor according to claim 1, wherein the receiving device comprises a plurality of receiving devices, and the vibration isolator has a honeycomb structure to partition the inner space of the housing into a plurality of spaces, the transmitting device is placed in a central one of the plurality of spaces, each receiving device is placed in a corresponding space other than the central one, and a distance from each receiving device to the transmitting device is the same. 